Comparison of UV Degradation of Polyethylene in Accelerated Test

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Downloaded by UNIV OF PITTSBURGH on August 16, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch037

Comparison of UV Degradation of Polyethylene in Accelerated Test and Sunlight Pieter Gijsman, Jan Hennekens, and Koen Janssen DSM Research BV, PO Box 18, 6160 MD Geleen, The Netherlands Studies of aging of polyethylene

in an accelerated

(Xenon) test and

outdoors in Geleen, The Netherlands, showed unexpected differences at the same degree of oxygen uptake. In outdoor weathering,

about twice

the oxygen uptake was necessary to give the same drop of the elongation at break and to form the same amount of carbonyl groups and unsaturation as in the accelerated weathering. by assuming different contributions by charge-transfer accelerated

complexes and from

and outdoor weathering.

The results may be explained

to oxygen uptake from the propagation

A possible improved

initiation

reaction for accelerated

test is discussed.

PoLYOLEFINS

DEGRADE UNDER T H E INFLUENCE of UV-light, which C a n

lead to failure of articles in outdoor applications (1,2). Determination of this UV-stability is a problem. In general, outdoor aging is too slow to be useful in the development of stabilizer formulations or for quality control. This flaw led to the development of several accelerated weathering tests (e.g., WeatherOMeter, Xenontester, Suntester, U V C O N , Q U V , and S E P A P ; ref. 2). Most of these accelerated weathering devices show poor correlation between the stabilities measured with them and those measured outdoors (3-6). Recent Fourier transform IR (FTIR) studies (7) suggested that this lack of correlation may be due to differences in degradation mechanisms in accelerated and out door testing. The most direct way to study the oxidation of polymers is to determine the rate of oxygen uptake by the polymer, but this method is experimentally difficult. In many studies it is done by measuring the drop in pressure in a closed system (8, 9). In such experiments the drop of the pressure is assumed to correspond quantitatively to the consumption of oxygen. This assumption can lead to errors if gaseous oxidation products are formed. The problem can

0065-2393/96/0249-0621$12.00/0 © 1996 American Chemical Society In Polymer Durability; Clough, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

POLYMER DURABILITY

622


be overcome if the amount of oxygen left is determined after each degradation period (10, 11). This chapter describes a study of the photooxidation of polyethylene (PE) in an accelerated (Xenon) test and i n outdoor weathering in Geleen, The Netherlands. The degradation was followed by measuring the true oxygen uptake by determination of the reduction of the amount of oxygen in the gas phase, along with changes in the F T I R spectra and the mechanical properties. The combination of these techniques led to new insights.

Experimental Degradation studies used 150-μπι thick, blown, low-density P E [ M 91,000; number of branches (CH ) per 1000 carbons = 20, amount of unsaturation (C=C) per 10 carbons = 55] films, designated ΡΕ I and ΡΕ II. Processing was done in the presence of a stabilizer, which was then extracted with refluxing chloroform. ΡΕ I was extracted for 25 h and ΡΕ II for 150 h. Both types of aging test were performed in a closed Durethan glass system with flat windows facing the fight source. This type of glass is transparent to fight with a wavelength above 290 nm. All experiments were done with air containing 0.83% helium to check possible leakages. Accelerated weathering was done in a Suntester (Hanau; filtered xenon lamp; intensity at 340 nm, 0.3 W/m ). The temperature was measured continuously in side the sample cells and was always in the 30-40 °C range. Sunlight exposures were done in Geleen, The Netherlands, by using plaques facing south at an angle of 45°. Outdoor weathering was started on 29 January 1989 for ΡΕ I and on 14 February 1989 for ΡΕ II. The temperature both outdoors and in the closed system was measured continuously and varied with the season (Figure 1). Oxygen uptake was determined by periodic gas chromatographic analysis of the gas phase Tby using the method previously described (II). Chemical cnanges were recorded oy FTIR analysis. Absorptions were calculated as the difference between the peak absorption and the absorption at a baseline. For the absorptions at 1712 and 1642 c m , the baseline was drawn between 1840 and 1600 c m ; for the absorption at 908 c m , the baseline was drawn between 950 and 860 c m . In addition to the chemical changes, the impairment of the mechanical properties was determined by using the elongation at break as a criterion (in absolute per centages). w

3

5

2

-1

-1

-1

-1

Results Accelerated and sunlight weathering of the polymers resulted i n an impair ment of the mechanical properties (Figure 2). For the accelerated weathering the drop of the elongation at break began after 1500-2000 h exposure; within 3000 h the polymers became totally brittle. For the samples weathered out doors the reduction of the elongation at break started after 12,000 h, and after 20,000 h exposure all polymers were brittle. The time until the elongation at break dropped to 50% of its original value was 6-8 times longer for the out door weathering than for the accelerated test.

In Polymer Durability; Clough, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

37.

GiJSMAN ET AL.

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UV Degradation of ΡΕ

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Figure 1. Mean monthly outdoor temperature as a function of exposure time inside (solid line) and outside (broken line) the closed system. 1000 so 0

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Figure 2. Elongation at break (%) as a function of exposure time (h) for accelerated weathering ofPEI(A) and ΡΕ II (O) and for outdoor weathering ofPE I (Π and +) and ΡΕ II (O). During accelerated weathering the oxygen uptake started immediately and was almost linear with time (Figure 3). During the first 1000 h the oxygen uptake was comparable for the two PEs, but after 1000 h a small deviation was found. After 3000-4000 h of accelerated weathering the polymers had an oxygen uptake of almost 1 mol/kg. During outdoor weathering the oxygen uptake curves showed totally dif ferent behavior (Figure 3). Exposure of these samples started in January and



624

POLYMER DURABILITY

Ο

5

ΙΟ

15

20

25 (Thousands)

Time (hours)

Figure 3. Oxygen uptake (mmol/kg) as a function of exposure time (h) for accelerated weathering ofPEI(A) and ΡΕ II (O) and the outdoor weathering ofPE I ( D and +) and ΡΕ II (O). February and the samples began to take up oxygen in the early spring. During the summer the rate of oxidation became constant. It slowed down at the end of the summer. During autumn and winter there was almost no oxygen uptake and in the early spring a second increase of the oxidation rate took place. After 22,000 h of outdoor weathering the total oxygen uptake was about 1.5 mol/kg for all three samples. The reproducibility of the outdoor weathering data was good. Changes in the IR spectra were also recorded during the degradation. For accelerated weathering the increase of the carbonyl absorption (1712 c m ) was almost linear in time, and it reached 0.8 after about 3000 h of degradation (Figure 4). In outdoor weathering the development of the car bonyl absorption is different (Figure 4). However, the shape of the curves is comparable with the oxygen uptake curves (Figure 3). After 20,000 h the carbonyl absorption was approximately 0.5. Changes in the amount of unsaturation during weathering were also re corded. The changes of the concentration of total unsaturation (absorption at 1642 c m ) and of end unsaturation (absorption at 908 c m ) are shown in Figures 5 and 6, respectively. The two curves show the same form as the corresponding plots for carbonyl absorption. As expected, accelerated weathering was faster than outdoor weathering. However, for outdoor weathering during the summer period the rate of ox ygen uptake was only 2.5 times slower than for the accelerated weathering. The mean rate of oxygen uptake for three summer periods was 0.2 mmol/ (kg*h), and the oxygen uptake rate during the accelerated weathering was 0.5 mmol/(kg*h). The acceleration factor is even smaller if we take into account -1

-1

-1



37.

GIJSMAN ET A L .

625


10

15

25 (Thousands)

Time (hours)

Figure 4. Carbonyl absorption at 1712 cm' as a function of exposure time (h) for accelerated weathering of PE 1(A) and ΡΕ II (O) and the outdoor weathering ofPE I (Π and +) and ΡΕ II (O). 1

Ο.ΘΟ h

(Thousands)

Time (hours)

Figure 5. Concentration of unsaturation (absorbance, 1642 cm' ) as a function of exposure time (h) for accelerated weathering of ΡΕ I (A) and ΡΕ II (O) and the outdoor weathering of ΡΕ I (Π and +) and ΡΕ II (O). 1

that the outdoor degradation only took place during part of the day, whereas the accelerated degradation was continuous. If outdoor degradation only oc curred for 10 h per day, then the outdoor oxidation rate in this period was comparable to that in accelerated weathering. The rates of formation of carbonyl and of end unsaturation during accel erated and outdoor summer weathering differed more than the oxygen uptake rates. For the accelerated test the rate of formation of carbonyl groups was 7.5 times higher than the mean formation rate over three summers of outdoor


626

POLYMER DURABILITY

weathering. Similarly, end unsaturation was produced 10 times faster in ac celerated weathering than in sunlight. The impairment of the mechanical properties was expected to be related directly to the oxidation of the polymer. Nevertheless, we found a difference between accelerated and sunlight weathering in the relationship between ox ygen absorption and the loss of elongation at break (Figure 7). For the ac-


1.00|

,

(Thousands)

Time (hours) Figure 6. End unsaturation (absorbance, 908 cm' ) as a function of exposure time (h) for accelerated weathenng ofPEI(A) and ΡΕ II (O) and the outdoor weathenng of ΡΕ I (• and +) and ΡΕ II (O). 1

1000

Ο

400

800

1200

1600

Oxygen uptake (mmol/kg) Figure 7. Elongation at break (%) vs. oxygen uptake (mmol/kg) for accelerated weathering of ΡΕ I (A) and ΡΕ II (O) and the outdoor weathenng of ΡΕ I (• and+) and ΡΕ II (O).


GIJSMAN ET A L .


37.


627

celerated weathering the drop of the elongation at break started at an oxygen uptake of about 350 mmol/kg, whereas for the sunlight weathering an oxygen uptake of 700 mmol/kg was observed before any significant loss in elongation at break was measured. It is interesting to compare the relations between oxygen uptake and the formation of the different products for the accelerated and the sunlight weathering. Accelerated tests can be reliable only if these relations are the same for both conditions. The IR spectra for samples having an oxygen uptake between 350 and 450 mmol/kg are shown in Figures 8 and 9. Even though the samples had taken up about the same amount of oxygen for the accelerated and sunlight weathering, the differences are remarkable. The oxygen uptake led to larger changes of the IR spectra for the accelerated weathering than for the outdoor weathering. The relationship between carbonyl formation and the oxygen reacted with the polymer depended on the kind of exposure. The conversion of oxygen into carbonyl groups was higher for the accelerated than for the outdoor weathering (Figure 10). The same effect was found for the relation between the oxygen uptake and the IR absorptions at 1642 and 908 c m (Figures 11 and 12, respectively). These figures reveal the difference in degradation chem istry during accelerated and outdoor weathering. Because mechanical property measurements are destructive, a large amount of polymer had to be exposed to provide enough material to allow determination of the time until mechanical failure. In literature studies it is common to assume a relationship between the amount of carbonyl groups formed and the changes in mechanical properties (3). The carbonyl groups can be measured using a nondestructive method such as IR so that only a small amount of polymer has to be exposed to determine the stability. This method has led to an increased use of the rate of change of the IR absorption at 1712 c m to determine the stability of a polymer. A plot of the elongation at break versus the carbonyl absorption (1712 cm" ) shows that this relation ship is not as universal as expected and depends on the kind of exposure (Figure 13). During outdoor exposure an increase of the carbonyl absorption led to a larger drop of the elongation at break than in the accelerated expo sure. The relationship between the amount of end unsaturation (IR absorption at 908 c m ) and the elongation at break is much less dependent on the kind of exposure (Figure 14). Thus, it is better to use the IR absorption at 908 c m to determine the stability of a polymer instead of the change of the IR absorption at 1712 c m . - 1

- 1

1

-1

- 1

- 1

The relationships between different IR absorptions developing during weathering also depend on the kind of weathering. The relationships between IR absorptions at 908 and 1712 c m are plotted in Figure 15. The ratio of end unsaturation to carbonyl groups is higher for the outdoor than for the accelerated weathering. - 1


POLYMER DURABILITY


628

Figure 8. IR spectra of PE exposed to accelerated weathenng (ΡΕ I and ΡΕ I before and after taking up 395 mmol/kg (ΡΕ I, top) and 359 mmol/kg (ΡΕ I bottom) of oxygen.


GIJSMAN ET A L .


629


37.

Figure 9. IR spectra of PE exposed to outdoor weathenng (ΡΕ I and ΡΕ II) before and after taking up 377 mmol/kg (PE I, top), 400 mmol/kg (ΡΕ I, midd and 420 mmol/kg (ΡΕ II, bottom) of oxygen.



630

POLYMER DURABILITY

0.0 400

ΘΟΟ

1200

1600

Oxygen uptake (mmol/kg)

Figure 10. Carbonyl absorption at 1712 cm' vs. oxygen uptake (mmol/kg) for accelerated weathering ofPE I (A) and ΡΕ II (O) and the outdoor weathenng ofPE I (Π and +) and ΡΕ II (O). 1

0.80

o.oo

400

800

1200

1600

Oxygen uptake (mmol/kg)

Figure 11. Unsaturation (absorbance, 1642 cm' ) vs. oxygen uptake (mmol/kg) for accelerated weathering ofPE 1(A) and ΡΕ II (O) and the outdoor weathenng ofPE I (• and +) and ΡΕ II (O). 1

Discussion Environmental factors such as light intensity, spectral distribution, and tem perature have an influence on the degradation rate during exposure of a poly mer. Differences in these factors can lead to differences in oxygen uptake, changes of the IR spectra, and impairment of the mechanical properties. A good correlation between an accelerated test and outdoor aging will be found


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Comparison of UV Degradation of Polyethylene in Accelerated Test

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