LIGHT AGING OF A POLYBLEND FILM UNDER INTERFERENCE FILTERS W A L T E R C. W A R N E R A N D E L B E R T E. G R U B E R General Tire and Rubber Go., Akron, Ohio
A light-aging study was undbrtaken to find a suitable means to evaluate the stabilization of a mixture of poly(viny1 chloride) resin and a copolymer of 1,3-butadiene with 2-methyl-1 -butene-3-one. Five interference filters provided a narrow window between 330 and 41 0 mp. A portion of film was mounted under each and exposed under a heliostat to Florida sun for approximately 2 months. After exposure, the stiffness of each portion of film was measured by the cantilever beam method. Wavelengths below 360 mp had a large stiffening effect; those from 360 to 390 mp had a small effect. The technique is unique, in that it permits the study of changes of physical properties attributable to the incremental effect of ultraviolet light. It could b e broadly applied to studying the aging of rubber and plastic films.
HE literature ( 8 ) reveals great interest by polymer chemists Tand technologists in the techniques and theory of ultraviolet light degradation of polymers. I t is difficult to isolate the effect of wavelength because many chemical and physical changes are occurring simultaneously. Some of the parameters involved are the ultraviolet absorption spectra of the various polymer systems, heat, oxygen, moisture, and cornpounding ingredients. I n addition, the distribution of wavelengths in the incident light must be considered. Testing techniques have been evolved using xenon as a n k h t - a g i n g SOUrce to sunlight more (7, 4 ) , the photochemical spectrometer to determine optically the activation spectra of polymers ( 5 ) ,glass absorption filters ( 6 ) , and mirrors to increase the total solar intensity (7). Practical answers to problems of ultraviolet stabilization in unfilled systems have usually involved the use of ultraviolet light screeners ( 7 , 2 , 3 ,.5,6,8).
Experimental
T h e polymer blend used for this study consisted of 50 parts of a calendering grade of poly(viny1 chloride) resin (PVC having a n intrinsic viscosity of approximately 1.1 deciliters per gram
Table I.
Emulsion Polymerization Recipe of Ketone Rubber
Ingredient
Parts by Weight 60 40 2
1,3-Butadiene 2-Methyl-1-butene-3-one Na dodecylbenzene sulfonatea Na lauryl sulfateb Potassium persulfate Mixed tertiary mercaptans Water Santomerse D . -95%.
Ingredient
Poly(viny1 ch1oride)a Ketone rubber Tin-ox gen polymer with butylside groups* 4,4 '-Thiobis( 6-tert-butylm-cresol). Ethylene distearamided a
c.
Calendering grade.
b
The interference filters (Baird Atomic, Inc., 35 University Road, Cambridge, Mass.) used consist of two parallel pieces of glass separated at a fixed distance by a stable natural resin. T h e inside faces of the glass are lightly silvered. Thus, most of the solar radiation striking the filter is absorbed by internal reflection. T h e significant exception is that a narrow band is transmitted, having peak at an exact wavelength determined bv the distance between the Dlanes of silver. T h e transmission curves of the interference filters used are shown in Figure 1. Since higher harmonic wavelengths of the desired peak are passed as well, it was felt desirable to superimpose a glass filter (Corning Glass Works, Corning, N. Y . )which transmits in the ultraviolet but absorbs in the visible. The transmission curves of the glass filters are shown in Figure 2. Portions of film were mounted on hardwood panels measuring approximately 3 X 6 inches by stapling a piece of film to the wood. T h e glass ultraviolet-transmitting filter was
1
D
E
2
'/Z
180
Temperature 122" F .
Table II.
in cyclohexanone a t 25' C.). The PVC was plasticized with 50 Parts of a copolymer consisting of 60 parts by weight of 1,3butadiene and 40 parts of 2-methyl-1 -butene-3-one. This polymer blend has the desirable properties of low volatility, low migration, and low lacquer lift. PvC plasticized with low molecular weight materials has been deficient in these Properties. The Polymerization recipe for Preparing the rubber and Of the polyblend are shown in Tables I and 11, respectively. T h e dry rubber crumb and pvc were mill-mixed a t 3000 F, for 10 minutes and calendered to a 4-mil film.
b
Dupanol C.
Conversion
Polyblend Compound
Parts by Weight 50 50
Advance 3.
Function
Resin Plasticizer
2
Stabilizer
2
Antioxidant Lubricant
2 c
Santowhite crystals.
d
Acrawax
300
I
i
,
350
400
450
WAVE LENGTH, rnp Figure 1 .
Transmission peaks of interference filters '401. 5
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SEPTEMBER 1966
219
mounted over the film and the interference filter on top. The filters were held in place by strips of manila folders stapled firmly. A 2 X 2 inch area of film was thus available for exposure. An exploded sketch of the filter assembly is shown in Figure 3. The data on the complete filter assemblies are shown in Table 111. The filter assemblies were sent to the South Florida Test Service and exposed for 195 to 260 ultraviolet sun-hours (2 to 2l/2 months, respectively). The longest tests were started first, so they would all be completed at the same time and minimize complications of dark reactions occurring subsequent to light exposure. The hours of exposure to sunlight were weighted inversely according to transmission of the combined filter assembly. The South Florida Test Service employed a heliostat, which is a motor-driven rack automatically facing the sun perpendicularly at all times. Upon return, the transmission curves of the filters were checked and found to be reasonably unchanged. Stiffness tests were run by the method of Newton and Wake (7),as shown in Figure 4. I n this method a strip of film is supported as a cantilever beam and allowed to deflect under its own weight as shown in Figure 4. The length is chosen so as to result in a deflection of 5 to 25y0 of the length. The stiffness (bending modulus) can readily be calculated from the observed length, deflection, thickness, and density. The data are presented in Table IV.
Discussion
By the technique of exposing films under interference filters, a marked increase in stiffening was found to occur as wavelength is decreased below 360 mp. T h e results are plotted in Figure 5. I t is interesting to compare the results of the technique used in this study with the conclusions of other workers in the field. Fundamentally, in all studies of the effect of ultraviolet wavelength upon properties, two independent effects are operating. The ultraviolet region of sunlight at the earth’s surface is generally considered to occur from 290 to about 400 mp (4). Wavelengths below 290 mp are stopped by the earth’s 100
r
atmosphere. Wavelengths above 400 mp are visible. As wavelength in this region is increased, the intensity increases. As wavelength is decreased, the energy per photon of light increases. The over-all destructive effect results from a balance of the separate effects of intensity and energy per photon. Thus, we can conclude from Figure 5 that the stiffening effect increases markedly as wavelength decreases below 360 mp. Further, since the lower limit of the solar spectrum a t the earth’s surface is 290 mp, we can infer that a wavelength causing maximum degradation occurs between 290 and 330 Enclosed carbon arc machines were initially used in this study and found to correlate very poorly with outdoor aging. The reason for the poor correlation is explained by the fact that enclosed carbon arc machines produce a strong maximum a t 385 mp and are deficient below 360 mp. These observations can be compared with those of Hirt, Searle, and Schmitt ( 5 ) ,who developed the technique of the photochemical spectrometer. I n this device a beam of sunlight reflected from a mirror driven by a heliostat is refracted into components by a quartz prism. Each wavelength strikes the same portion of the specimen all day long. The range of wavelengths from 290 to 400 mp has been studied very thoroughly. Absorption spectra can then be obtained on the sample after exposure and the wavelength of peak damage determined. Hirt, Searle, and Schmitt found that many polymers have a region of maximum sensitivity to solar radiation in the range of 310 to 330 mp. This maximum is called the “activation spectrum.” For PVC it occurs a t 310 mp. The presence of a wavelength in the ultraviolet of minimum destructiveness has also been reported by Hirt, Searle, and Schmitt ( 5 ) . They found a double peak with the photochemical spectrometer with a vinyl chloride-vinyl acetate copolymer. T h e wavelengths of maximum degradation were 322 and 364 mp, respectively, with a minimum between.
~,~~
INTERFERENCE FILTER
7.54
5.57
HARDWOOD
400
600
500
200
300
Figure 2.
Ultraviolet-transmitting glass filters
WAVELENGTH, rnp
Table 111.
Interference Filter Designation
Peak, mp Factory numbera Half peak width, mp 7’ transmission at peak Before exposure After exposure Glass filter numbera 7’ transmission at Baird peak Assembly Yo transmission UV sun-hours of exposure a
220
Baird-Atomic, Inc.
b
Figure 3. Exploded sketch of filter assemblies
Interference Filter Assembly Data
A
B
c
330 7-1870-A 20
347 7-3726-4 18 27.8 22.8 7-54 87 24.2 195
25.0 19.2 7-54 89 22.2 210
Corning Glass Works.
I & E C P R O D U C T RESEARCH A N D DEVELOPMENT
367 7-3753-3 36
D 395 7-3074-4 30
E 410 7-3035-4 18
22.0 27.4 5-57 79 17.4 260
21.7 41.2 5-57 84 18.2 260
22.0 36.2 5-57 83 18.2 260
Possible Applications of Interference Filter Technique Table IV.
Degradation of Film as a Function of Wavelength Interference filter A B C D E
Peak, mp Original stiffness‘l Exposed film stiffnessa
330 6
347 6
367 6
395 6
410 6
90 51 23 20 Stt’Jness units, bending modulus in thousands of p . s . i .
28
T h e use of interference filters has some intriguing possibilities which would complement studying solar degradation of polymers by optical means. T h e 2 x 2 inch area available makes other tests possible, such as tensile strength, or elongation on a small scale. Nondestructive physical, optical, or electrical properties could be evaluated prior to destructive tests. When this study was undertaken, the 330-mp filter had the lowest wavelength peak available. Now lower wavelength filters are commercially available, which means that the solar region from 300 to 290 mp could be studied. Conclusions
1.5
E =
x
106d~4
b2Y Figure 4.
Stiffness by the cantilever beam
E. d. 1.
Stiffness (bending modulus), p.s.i. Density, Ib./cu. ft. Overhanging length, inches b. Thickness, mils y. Deflection, inches
50C
250
100
The contributions of various portions of the ultraviolet spectrum of sunlight to the aging of polymers have been isolated through the use of interference filters. T h e filters provide a n area of exposure large enough for physical tests (2 X 2 inches). T h e technique has a n advantage over the photochemical spectrometer in that many specimens can be exposed simultaneously. I t has the advantage over absorption filters in that the incremental effect of wavelength in the ultraviolet can be correlated with physical, optical, or electrical properties of polymer films. I n the polyblend studied, the wavelengths of maximum destructiveness were found by the interference filter technique to be below 360 mp, which correlates well with other studies (5, 6 ) . I t would be of interest to study this range in greater detail with interference filters which are currently available. I n this way, it is expected that a wavelength of maximum destructiveness could be accurately located for many polymer systems. Similar results would be expected using the more common polyblend of PVC plasticized with butadieneacrylonitrile rubber.
50
Acknowledgment
25
T h e authors thank W. L. Kollar, who performed some of the measurements, and The General Tire and Rubber Co. for the support of this work.
UV
--I+
VISIBLE
10 ORIGINAL FILM 5 300
I
1
350 400 PEAK WAVE LENGTH, rnp Figure 5. Stiffness vs. wavelength
An interesting comparison can be made with the technique of Melchore ( 6 ) ,who used a series of glass filters, each having a different cutoff point in the ultraviolet. By observing polymer embrittlement and carbonyl formation as a function of wavelength, he concluded that wavelengths most destructive to polypropylene are in the vicinity of 300 mp.
literature Cited
(1) Garner, B. L., Papillo, P. J., IND.ENC. CHEM.PROD.RES. DEVELOP. 1,249-53 (1962). (2) Gruber, E. E., Warner, W. C., U. S. Patent 2,786,044 (March 19, 1957). ( 3 ) Gysling, H., Heller, H. J., Kunststofe ( M u n i c h ) 51, 13-17 (1961). (4) Hirt, R. C., Schmitt, R. G., Searle, N. Z . , Sullivan, A. P., J . Opt. SOC. Am. 50, 706-13 (1960). (5) Hirt, R. C., Searle, N. Z., Schmitt, R. G., SPE Trans. 1, 1-5 (1961). (6) Melchore, J. A,, IND.ENC. CHEM.PROD.RES. DEVELOP.1, 232-5 (1962). (7) Newton, R. G., Wake, W. C., J . Rubber Res. 19, 9-16 (February 1950), Rubber Chem. Technol. 24, 1-17 (1951). (8) Searle, N. Z., Hirt, R. C., SPE Trans. 2, 1-23 (1962).
RECEIVED for review February 1, 1965 ACCEPTED June 15, 1966 Division of Rubber Chemistry, 148th Meeting, ACS, Chicago, Ill., September 1964.
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