ACCELERATED OUTDOOR EXPOSURE TESTING I N EVALUATION OF ULTRAVIOLET LIGHT STABILIZERS FOR PLASTICS B. L. GARNER AND P. J . PAPILLO Geigy Industrial Chemicals, Division of Geigy Chemical Carp., Ardsley, N .
Y.
The goal of most accelerated exposure testing is prediction of resistance to weathering. Because results of such tests frequently fail to correlate with long-term exposures, methods have been designed to present to the test piece radiant energy with spectral distribution more closely approximating that of sunlight. One device, the EMMA, intensifies natural sunlight through a combination of mirrors and equatorial mount. Samples are cooled b y forced air. Although sunlight intensity i s increased tenfold, acceleration of degradation ranges from twofold to elevenfold, depending on polymer and formulation under test. Except for thermally unstable resins, which give erratic results, ranking of weatherability of formulations of a single polymer system is good. The relative value of light stabilizers may b e predicted, but without extensive experience with the polymer type under test, long-term exposure tests are required for quantitative evaluations.
HE PRESENT STATE of the art of polymer design and formuTlation necessitates the use of various stabilizers to upgrade resins intended for commercial applications. Without proper stabilization most commercial polymers would be severely limited, not only in their ability to be fabricated but also in their scope of application. L2‘ith the continued demand for shorter processing times and the growing application of plastics in architectural, automotive, and other exterior fields, there is a concurrent demand for greater resistance toward the degradative effects of heat and weathering-i.e., oxygen, moisture, light, etc. Those engaged in meeting this demand require a knowledge of the stabilization imparted by various materials at once and not after 1 to 5 years of in-use testing; consequently, accelerated testing which will accurately (preferably quantitatively) predict the relative efficacy of such materials is needed. This paper examines the usefulness of a specific accelerated outdoor exposure device for evaluation of the light stability of plastics. Stabi1it)- to light may be improved through incorporation of pigments. antioxidants. and ultraviolet stabilizers ( 6 , 7, 72, 73). For the most part, these ultraviolet stabilizers strongly absorb ultraviolet light and have a chemical structure that is unusually stable to the absorbed energy. Since they function primarily by absorbing actinic radiation and their optical characteristics are readily determined, it has been suggested that with a knowledge of the response of a given polymer to light of each wavelength, the effectiveness of light stabilizers for that polymer might be predicted (4,77). The mathematical basis for such calculations is well established, and some knowledge of the wavelength by wavelength response of polymers has been developed ( 9 ) . However, ultraviolet light absorbers are frequently used in conjunction with antioxidants and orher additives which contribute to ultraviolet stability (3) and there is evidence that the supposedly stable ultraviolet
absorbers demonstrate some degree of antioxidant activity ( 5 ) . Problems of compatibility, volatility, and sensitivity to metals or alkalinity further complicate evaluation of light stability, so that calculation of effectiveness is impractical as a standard procedure. Many types of accelerated tests for the determination of light stability of plastics have been developed, based upon various sources-carbon, mercury, and xenon arcs, fluorescent sunlamps and black lights, or other ultraviolet-rich devices, sometimes in combination with filters. In recent years the difficulties encountered in translating the results of such accelerated tests to useful prediction of life under service conditions have become increasingly evident. The development of the xenon arc exposure devices-Le., the Xenotest and the xenon arc Weather-Ometer-represents one effort to match more closely the energy distribution and intensity of average sunlight. Test results are accelerated through 24hour-per-da) exposure to “noon” sunlight, rather than intensification of exposure condition. \\‘ith standardization of lamps and filters, and with appropriate compensation for aging characteristics of both, the xenon arc exposure device should become the standard laboratory exposure method of the near future. Another concept of accelerated testing. advanced by Caryl and Helmick ( Z ) , involves the use of natural sunlight intensified by mirrors in combination with a n equatorial mount. Forced air cooling minimizes overheating of samples. The usefulness of this device, knovm as an EhISIA (Figure I), is examined here. The EMMA employs ten highly polished Alzak-treated aluminum mirrors to increase natural sunlight intensity on the sample. The mirrors, electrolytically brightened and coated with a 0,0002-inch-thick transparent oxide film, show relatively minor dropoff of reflectance in the ultraviolet region of VOL. 1
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DECEMBER 1 9 6 2
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Figure 1.
EMMA, mirror exposure device
interest, as is illustrated by Figure 2. I n practice these mirrors are cleaned daily, and are replaced at the first sign of surface degradation. The energy distribution of sunlight which is incident upon samples on the EMMA varies throughout the day and thraughout the year, as does natural sunlight. The similarity of the energy distribution to that of natural sunlight is seen in Figure 3, which compares a typical summer sunlight energy spectrum (8) to that calculated far intensified sunlight. The daily accumulation of ultraviolet by the samples is further increased by the equatorial mount. Figure 4 illustrates the hourly sunlight flux received by a sample on an equatorial mount, as compared to that received by a sample mounted 45' above horizontal and facing south. These data far June 14, 1961, a t Phoenix, Ariz., show an increase in langleys from 587 to 972, a 61% increase in total langleys through the use of the equatorial mount, and a 900% increase to 5900 langleys with the EMMA. The langley, or gram-calorie per square centimeter, as a unit of exposure has the disadvantage that not ultraviolet but total sunlight energy is measured. The advantages and disadvantages of this "best available" exposure unit have been discussed (7). Sample temperature on the EMMA is somewhat higher than at 45' S, hut this appears to be a limitation in only a few cases. Table I gives black panel and white panel temperatures measured in August on an EMMA and at 45' S in Phoenix. Although black panel temperatures of 170' F. _"the P L o . f i d "'Lf Lo "..*:":-"*-A c -.. -.* >..L A L_1-.-yi*Ai*l'. .,L LLLL'C.pLLL" I U L C"L'L"'C,y U"L US>CII 1
-_1.
LL.c
days, on the same days black panel temperature at 45" S may be nearly as high, 160' F. Actual sample temperatures, of course, would be much leu. The present report correlates the degree of deg-radation of samdes exDosed on the EMMA with that of identical samdes e xposed a t 45" S. Degradation was quantitatively assessed by determining the reflectance of white-backed samples with a €Iunter Color and Color Difference Meter, Model D25; the ,:-,.A-..., a- m~ oI~ , 1 ~ 1 1 1 . .> L , (ngrr~nrss, (yeuow-mue aspecr) values were n e t u mined. The formula whiteness = W = L - b was taken as reasonably representative of the sample ranking as seen by the eye. The more generally accepted formula, W = L - 3b (70),was developed far near-white samples and gives erratic ranking with dark samples. I n general, comparisons among samples involve exposure periods to the same extent of degradation. and onlv comoarison of discoloration of differins hue may be seriously questioned. No such samples are inc luded in this report. Data are available for three resin systcms exposed both .L^ n l l l b * ._I ._ ~ ~ ~" 7 0 ~n 7 -1 011 YK L / J V U V L ~ diiu u u cxpururt. i a c ~ sar 4 3 3. r n e~ nmrsystem consists of glass-reinforced chlorinated polyester, the second of rigid PVC stabilized with a tin mercaptide, and the third of rigid PVC stabilized with a barium-cadmium-phosPhite combination. Each resin system consists of samples ~~~
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Table I.
Time 8
A.M.
10 A.M. Noon 2 P.M. 4
P.M.
Temperatures (" F.), August 12, 1960 EMMA 8, 45" South Block Black Whit; Ambient Pallel panel pone1 Air
134 151 154 166 150
123 137 155 142 128
98 113 120 119 114
95 102 106 110
110
~
~
Sola Radiatm n Roenx S t a t t m y Rack 45" South 587 langleys 6 3 sm hwn
SUNLIC INTENS
Solar Radiatm in phoenix Follow the son rack 6EK) 972 langleys 11 8 Sun Hous on EMMA and EMMAWA machines
AFT E RNOON
Figure 4.
TIME
MORNING
Time vs. energy as received b y samples on an equatorial mount and a t 45" s
xt-ithout ultraviolet absorber; with the polyester and tin mercaptide-stabilized PVC, several levels of one or more ultraviolet absorbers are included. Duplicate sets of samples of each formulation were exposed under each of the t\vo exposure conditions. Resultant L - b values were plotted against exposure time for each formulation. The raTes of yellowing for 45' S and EMhlA exposure are presmted in Figure S for the three resin systems without ultraviolet absorber. It is apparent that the relation between rate of yellohving under the two exposure conditions is very much a function of the formulations under test. Figure 6 is a correlation plot of these same data. For each resin system the langleys to a given degree of yellocving on the E M M A are plotted against the langleys required at 45' S to give the same yellowing. The diagonal straight lines represent the relative degradative effect of equal numbers of langleys under each of the t\vo exposure conditions. The diagonal labeled 1.0 indicates that the langleys received by a sample on the EMMA cause the same degree of discoloration as the same number of langleys received at 45' S. However, the 0.5 diagonal indicates that EMMA langleys cause only 50y0 the discoloration of langleys received a t 45' S. It is apparent from Figure 6 that the relative degradative effect of a langley not only is a
function of the formulation, but varies \vith the extent of the exposure as well. This decrease in exposure efficiency for the accelerated test condition is not surprising when it is considered that many nonphotochemical factors play a major role in pol>-mer degradation. Temperature, oxygen availability, and moisture can play a prime role and yet are not accounted for in the langley unit. An exposure efficiency o l 50% on the EMMA still represents a fivefold acceleration in time compared with exposure a t 45' S. The introduction of ultraviolet absorber into two of the resin systems is illustrated in Figures 7, 8. and 9. I n each instance one or more concentrations of 2-(2 '-hydroxy-5'-methylphenyl)benzotriazolr were added. and in the case of the chlorinated polyester, a representative formulation \sith 2-hydroxy-4methoxybenzophenone also was included. The correlation data of Figure 10 show that the exposure efficiency is a function not only of the base resin, but of the type and quantity of additive, such as ultraviolet absorber or thermal stabilizer. This dependence of acceleration upon formulation prohibits the direct quantitative transformation of EMMA data to 45' S data, a more comfortable base for the prediction of aging characteristics. \Vith this in mind two questions concerning EhIM.4 exposures may be asked: To what extent do they yield VOL. 1
NO. 4
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251
’-
80
t EMMA
- --
1
90
t EMMA
- --
ABSORBER CONCENTRATIONS AS INDICATED
70 A
I
2 vi
RIGID PVC WITH TIN MERCAPTIDE
60 W
k B
M
40
30
-
20
-
10
‘-
‘\?>
CHLORINATED POLYESTER
\ \
\---
I
2o
lo 0 KILOLANGLEYS 0 DAYS EMMA 0 DAYS 45‘5 0
120
60 13 115
180 41 331
27 225
240 53
300 64
360 81
Figure 5. Yellowing of three resin systems upon exposure on EMMA and at 4 5 ” S
useful information and what additional information is needed to obtain more quantitative predictions from the EMMA? Tabulated in Table I1 are the langleys required to obtain a specific amount of degradation of color with several resin formulations. A value of W = 55 was taken as failure for the chlorinated polyester and W = 7 2 for rigid PVC with tin mercaptide. The extent of yellowing chosen-Le., W = 55represents an arbitrary “failure” point: a yellowing sufficient to be considered undesirable even though no loss of physical properties is apparent. The percentage improvement in time to failure varies considerably with the type and quantity of
It
0 KILOLANGLEYS 0 D A Y S EMMA 0 DAYS 45’5 0
60 13 115
120 27 225
180 41 331
240 53
360
300 64
e1
-
Figure 7. Yellowing of chlorinated polyester with 2 4 2 ’-hydroxy-5 ’-methylpheny1)benzotriazole
ultraviolet absorber. However, the relative effectiveness of the two ultraviolet absorbers upon exposure a t 45” S is adequately predicted by the EMMA exposures. While hardly quantitative, the EMMA data give assurance that the same money spent on benzotriazole would give equal or better protection and increased life to the polymer. With the EMMA, as with many exposure devices, acceleration distorts test results. If exposures were obtained for a lesser
Table II. Exposure to “Failure” for Chlorinated Polyester and PVC Systems with Ultraviolet Absorbers 200
EMMA Exbosure 100
Kilolangleys to Failure EMMA 45’s
n W
dzv) $9
Chlorinated polyester, failure =L-b=5j No absorber 2-( 2 ’-Hydroxy-5’-methylpheny1)benzotriazole 0.7% 1.4% 2.0%
5o
- I -4 x
30
20
45 s/ EMMA),
6
8
133
65 105 132
35 74 110
54 71 83
96
53
55
40
6
15
74
29
39
2-Hydroxy-4-methoxybenzo-
10
10
20
30
50
100
100
KILOLANGLEYS ON EMMA
Figure 6. Exposure to equal yellowing for three resin systems EMMA vs. 45’ S 252
E&ciency (Rttio
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
phenone, 2.0% Rigid PVC (tin mercaptide), failure = L - b = 72 No absorber 2 4 2 ’-Hydroxy-5’-methylphenyl)benzotriazole, 0.35%
%
r-----l
loo
90
80
70
60
\ 0.54:
A
\
I
A
30
0 1
Figure 8.
-
0 0 0
I
I
I
bo
120
13 115
225
180 41 331
27
I
‘240 53
\
I-
30
20
KILOLANGLEYS DAYS, EMMA DAYS, 4 5 ‘ 5
c
40
40
20
-
10
-
0.35%
\
\
\
-I
\
\
0%
\
\
-
I
I
Mo
360
64
81
Yellowing of chlorinated polyester with 2-hy-
droxy-4-methoxybenzophenone
degree of intensification of sunlight as well as for tenfold intensification, the additional data might allow extrapolation to exposure a t 45’ S. .A series of test exposures is currently under way a t 45’ S, on the ten-mirrored EMMA, and on a special EMMA with only four mirrors, to test this hypothesis. I n addition, data of this type are being developed for various other resins, including unsaturated polyesters, polystyrene, and polyolefins. As now operated, the E M M A gives a reliable first indication of the stability of plastics to outdoor exposure. With additional experience, and with results from a four-mirrored weak sister of the EMMA, it may be possible to correct for the distortion of exposure results obtained with the accelerated test, thereby securing a reliable indication of the weatherability of resin formulations. literature Cited (1) Caryl, C. R., A. S. T. M. Bull. 243, 55-9 (January 1960). (2) Caryl, C. R., Helmick, W. E., U. S. Patent 2,945,417 (July 19,
1960). (3) Clark, G. A., Havens, C. B., Plastics Technol. 5,41-4 (December 1959). (4) Claudet, J. H., Newland, G. C., Patton, H. W., Tamblyn, J. W., SPE Conference, “Stability of Plastics,” Washington, D. C., Dec. 1, 1959. (5) Claudet, J. H., Tamblyn, J. W., SPE Trans. 1, 57-62 (1961). (6) Gottfried, C., Dutzer, M. J., J . Appl. Polymer Sci. 5 , 612-19 (1961). (7) Gysling, H., Heller, H. J., Kunststofe 51, 13-17 (January 1961). (8) Hiit, R. C., Schmitt, R. G., Searle, N. D., Sullivan, A. P., J . Opt. Soc. Am. 50, 706-13 (1960). (9) Hirt, R. C., Searle, N. D., Schmitt, K. G., S P E Trans. 1, 1 (1961).
KILOLANGLEYS DAYS, EMMA DAYS, 45’ 5
0
60
0
im
13 115
21 225
0
180 41 331
240 53
300 64
360 81
-
Figure 9. Yellowing of PVC-tin mercaptide with 2-(2’-hydroxy-5’-methylphenyl)benzotriazole
500 2 - ( 2 ’ - HYDROXY BENZOTRIAZOLE
- 5’-METHYLPHENYL)
NO ABSORBER
1
1
1
1
5
10
M
1w
500 1000
KILOLANGLEYS ON EMMA
Figure 10. Exposure to equal yellowing of chlorinated polyester with ultraviolet absorber EMMA
VI.
45’ S
(10) Hunter, R. S., J . Opt. SOC.Am. 50, 44-8 (1960). (11) Miller, C. D., 0 8 c . Digest Federation Paint Varnish Prod. Clubs 30, 612-19 (1958). (12) Rosa, P., Elm, A. C.,Zbzd.,31, (1959). (13) Zussman, H. W., Mod. Plastics, 1960, Encyclopedia Issue. RECEIVED for review September 27, 1962 ACCEPTEDOctober 10, 1962 Symposium on Stabilization of Polymers, Division of Organic Coatings and Plastics Chemistry, 142nd Meeting, ACS, Atlantic City, N. J., September 1962. VOL. 1
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