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7 Differential Ultraviolet Spectroscopy as an Aid in Studying Polycarbonate

Photodegradation

Downloaded by RUTGERS UNIV on March 11, 2016 | http://pubs.acs.org Publication Date: April 8, 1981 | doi: 10.1021/bk-1981-0151.ch007

J. E. MOORE General Electric Corporate Research and Development Center, Schenectady, NY 12301 The photochemical degradation of bisphenol-A polycarbonate (PC) has been the subject of a number of investigations.— As will be illustrated, our own studies on PC film weathering indicate that the major parameter which affects PC degradation is the amount of UV light to which the sample is exposed. This implicates photochemical reactions as principal degradation pathways. Artificial weathering normally accelerates the natural weathering process by using a more intense source of UV light. Care must be taken, though, that high intensities of UV light at wavelengths much shorter than those found in natural sunlight (less than 295 nm) do not give misleading results. A number of the previously cited investigators * ? " have employed UV spectroscopy as an analytical tool for following PC degradation. We have found the measurement of UV spectra of weathered PC films by difference from an unexposed reference sample to be an extremely simple and useful analytical method. This nondestructive analysis allows the repetitive return of a sample to the exposure conditions and thus enables one to essentially perform continuous analyses on the same sample. This technique, of course, will not detect the formation of nonchromophoric products such as aliphatic oxidation products which may form during the degradation. Previous work on PC photo-degradation mechanisms has mainly stressed the photo-Fries reaction which leads to the formation of substituted phenyl salicylates and dihydroxybenzophenones (Fig. 1). However, chain cleavage reactions which lead to phenolic products (Fig. 1) have also been reported to play important roles in PC weathering. The importance of oxygen on the photo-degradation of PC will be the subject of a forthcoming publication from this laboratory.^ PC photo-degradation has been monitored using outdoor exposure at Schenectady, NY and under accelerated weathering conditions using RS sunlamps as a UV source. 1

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0097-6156/81/0151 -0097$05.00/0 © 1981 American Chemical Society

Pappas and Winslow; Photodegradation and Photostabilization of Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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98

PHOTODEGRADATION

AND

PHOTOSTABILIZATION

O F COATINGS

E

I

OH

E

o

n


HO H

m

i

H ABSTRACT IQM

1

I** ]

• Y] 1

Figure

1.

Photo-Fries

J

— H ABSTRATION

2

~ @

0

H

-~H • HOO— \^/ ^ 211 n n

rearrangement and other polycarbonate

n

u

s

photoinduced

degradations of


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MOORE

Polycarbonate

Photodegradation

99


Experimental Measurements of light intensity at various wavelengths, of different light sources were made with a Model 585-66 EG&G spectroradiometer. Plots of intensity vs. wavelength for natural sunlight and the RS sunlamp are shown in Figure 2. Ten mil Lexan®*PC film containing no UV-stabilizers was obtained from Sheet Products Section, Mt. Vernon, Indiana. Reference films containing various additives were prepared by dissolving the additive and Lexan PC resin in dichloromethane and casting films from solutions containing about 5% solids. Portions of these films were selected which were 2 ± 0.1 mils thick. UV spectra were recorded using a Perkin-Elmer Coleman 575 spectrometer. Yellowness index measurements (ASTM-D-1925) of film samples were made using a Colormaster Model V colorimeter. Film samples were exposed to natural weathering conditions on an exposure rack constructed of wooden 2 X 4's. The samples were mounted directly on a 2 X 4 facing south at an angle of 45° to the horizontal. A measure of the UV light received by the samples was originally obtained from New York State Department of Environmental Conservation (ENC0N) reports, kindly supplied by Mr. William Delaware of ENCON. These reports included both total sun and sky radiation and UV radiation measured by Eppley radiometers (photometers). The ENCON monitoring station in Schenectady is within four miles of the exposure rack so no appreciable differences in light intensity should be expected. More recently, a UV radiometer has been mounted directly on the exposure rack to obtain more accurate measurements. Film samples were also exposed to UV light on RS sunlamp turntables. These consist of two RS sunlamps mounted 10 inches above 12 inch diameter turntables which were rotated at about 5 rpm. Alternate lamps were replaced every week. Results and Discussion Natural Weathering. In order to follow chemical changes in PC resulting from weathering, ten mil PC films were examined by differential UV spectroscopy at intervals during natural and artificial weathering. The spectral changes were correlated with changes in the yellowness index of the samples and with conditions to which the samples had been exposed. Measurements of solar UV-exposure at Schenectady during 1977 from ENCON data are shown in Table 1.

* Lexan® is a registered trademark of the General Electric Company.


PHOTODEGRADATION

100

A N D PHOTOSTABILIZATION

O F COATINGS

TABLE I 1977 Outdoor Weathering Data from ENCON Latitude Elevation

42"47'50" 340 ft.


Calories/Cm

January February March April May June July August September October November December 1977 TOTAL

Total

UV

% UV

4154 5096 8432 11280 15500 11820 14911 11470 7512 5580 3300 2975 112030

175.2 222.0 381.6 456.0 629.2 531.0 558.2 462.0 292.6 204.3 133.3 103.0 4142.6

4.07 4.36 4.53 4.04 4.06 4.49 3.74 4.03 3.90 3.67 4.04 3.46 3.70

Typical differential UV spectra are shown in Figure 3. If the sample and reference films were exactly the same thickness, there would be no difference in absorbance at any wavelength, and only a straight line at absorbance = 0 would be obtained. In Figure 3, the sample film is slightly thicker than the reference film, so there is a small peak at about 285 nm. This is caused by the inability of the spectrometer to measure differences in PC absorbance when the absorbance is very high. However, when the absorbance is lower, as it is at longer wavelengths, the spectrometer is able to measure the difference in absorbances of 10 mil PC films starting at about 285 nm. Experience has shown that the disappearance of background noise indicates the wavelength cutoff for differential UV spectra. This wavelength corresponds to an absorbance of about 3-4 on a UV spectrum of a film versus air. During exposure to UV light, there is initially a slight decrease in the peak at about 285 nm, followed by a regular increase in this peak. Simultaneously, there is the formation of a "negative peak" at about 310 nm in the early stages of exposure. A peak at about 340-360 nm also develops slowly during the exposure. The reasons for the initial decrease of the 285 nm peak and the formation of the negative peak at 310 nm are not completely clear. The phenomena are transitory but quite reproducable and real. An obvious explanation is that some chromophores in the exposed


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Polycarbonate

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Photodegradation

sample are being destroyed or "bleached" during the exposure. A decrease in the yellowness index (YI) of the exposed sample is also noted during this time. After the initial bleaching, there is a relatively rapid increase in the peak at 287 nm and slower increases in the peaks at 340-360 nm and 310-320 nm. A plot of the increase in these peaks as a function of UV dosage (cal/cm ) is shown in Figure 4. It is fairly certain that these peaks are due to phenolic, phenyl salicylate, and dihydroxybenxophenone decomposition products respectively as previously postulated.— Figure 5 shows UV spectra of 2 mil PC films containing one percent of these ingredients. It is apparent from the relative sizes of these peaks and the corresponding peaks in spectra of degradated PC that the major decomposition path for PC undergoing natural weathering is a chain scission reaction leading to products containing phenolic end groups. Evidence corroborating the formation of phenolic products in weathered PC is shown in Figure 6. In this figure, the differential infrared spectrum of a 10 mil PC film weathered for one year at Schenectady shows a strong phenolic peak at 3500 cm" . The identification of this peak is confirmed by the differential IR spectrum of a 2 mil PC film containing 5% BPA shown in Figure 7. Intrinsic viscosity measurements of original and weathered PC films indicate a decrease in the viscosity average molecular weight (M ) from 34,300 to 21,100 during the year of weathering. This confirms extensive degradation by chain cleavage reactions, which lead to phenolic products, since the photo-Fries rearrangement does not affect the polymer molecular weight. The formation of the major UV degradation peak at about 287 nm in the weathered PC appears to correlate well with the formation of the yellow color in the weathered sample. In Figure 8 the formation of both the peak at 287 nm and the yellow color have been assumed to be products of a first order reaction. This figure shows a plot of the log of the percentage of a scaling constant minus the yellowness index divided by the constant, versus a measurement of the exposure. In this case, the exposure is expressed as cal/cm , obtained from ENCON data. It is apparent from Figure 8 that the formation of the 287 nm peak and the yellow color are directly related and that both apparently follow first order kinetics.


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Accelerated Weathering. A number of instruments have been devised for accelerating natural weathering conditions. The instruments usually incorporate a source of UV light since i t is the UV which rapidly degrades plastic samples. One of the simplest and cheapest sources of UV light is the GE RS sunlamp. This has been used extensively for years to provide accelerating weathering conditions. The lamp consists of a medium pressure mercury arc which is ballasted by the tungsten filament incan-


102

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PHOTOSTABILIZATION

OF

COATINGS


PHOTODEGRADATION

Figure

4.

Changes in PC UV spectrum peaks during natural weathering nm; (A) 310-320 nm; (\Z\) 340-360 nm)

((Q)


287

MOORE

Polycarbonate

Photodegradation

0.8

1

1

0.7

1

1

1% * SALICYLATE IN 2mil PC FILM vs 2mil PC FILM

0.6

A 1% 2,2'-DHBP IN 2mil PC FILM \ vs 2mil PC FILM

0.5


0.4 l%BPAm 2mil 0.3 ~ PC FILM vs

. f

2mil PC FILM-L

0.2

| \\J

\

\

-

0.1 0

Figure

5.

Differential

1 250

1 350

i 300

i ' 400

UV spectra of 2-mil PC films containing

3,o .lOmil PC FILM vs lOmil PC FILM

various

additives

^

lOmil PC FILM WEATHERED I YEAR vs UNEXPOSED :o lOmil PC FILM

Figure 6.

IR spectra of 10-mil PC films


104

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PHOTOSTABILIZATION

OF

COATINGS


PHOTODEGRADATION

Figure

8.

Formation

of 287-nm peak and change in yellowness during natural weathering

index in PC


film


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Polycarbonate

Photodegradation

105

descent portion of the lamp. A measurement of the UV output of the RS sunlamp in comparison to natural UV light is shown in Figure 2. A rotating exposure table was used to maximize uniformity of sample exposure under the RS sunlamps. Typical differential UV spectra of 10 mil PC films exposed under RS sunlamps are shown in Figure 9. Again the major spectral changes caused by the UV light exposure are the formation of a sharp peak at about 287 nm and a broader peak at about 320 nm. The formation of the peak at 320 nm is in contrast to the "bleaching" in this area in samples exposed to natural light (compare Figures 3 and 7). The formation of the peak at about 320 nm is related to the light intensity at wavelengths below about 300 nm. As shown in Figure 2 the light intensity at 300 nm is about 20 times higher under the RS sunlamp than in natural sunlight. Yellowness indices of the exposed PC samples are easily measured in the colorimeter during the course of the exposure. A plot of the increases in absorption at 287 nm and in yellowness index (AYI) versus exposure time under RS sunlamps is shown in Figure 10 (as was shown for the same parameters with natural sunlight exposure in Figure 8). From these two plots, the points where the changes in absorption at 287 nm and in yellowness index are equal can be determined and compared with the exposures needed to effect these changes. This comparison indicates that one year of natural exposure at Schenectady is equivalent to 170 hours of RS sunlamp exposure when changes in absorption at 287 nm are measured and to 140 hours of RS sunlamp exposure when changes in yellowness index are determined. Conclusions 1) The use of differential UV spectroscopy is a facile analytical tool, providing a rapid, non-destructive method for determining the course and extent of degradation of PC films during accelerated or natural weathering. 2) Chain cleavage with subsequent formation of phenolic products, rather than the photo-Fries rearrangement to form salicylates and dihydroxybenxophenones, has been identified as the major initial degradation pathway of PC exposed to natural weathering conditions. 3) The spectroscopic method is also useful for correlating the rate of PC degradation under accelerated weathering to that under natural weathering conditions. Acknowledgements The author thanks the General Electric Research and Development Center for permission to publish this work and thanks Mr. S. T. Rice for many of the yellowness index and UV spectral measurements. Mr. William Delaware of the New York State Department of Environmental Conservation is also thanked for his data.


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PHOTODEGRADATION

Figure

10.

Formation

of 287-nm peak and change in yellowness index in PC film during exposure under RS sunlamps


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Polycarbonate

Photodegradation

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Abstract Films of bisphenol-A polycarbonate were weathered on an outdoor rack in Schenectady, NY and under accelerated conditions under RS sunlamps. Changes in the samples were measured by changes in yellowness index (ASTM-D-1925) and by changes in the UV spectra relative to an unexposed reference. The formation of phenolic degradation products followed by formation of photoFries products (salicylates and dihydroxybenxophenones) can readily be detected in the UV spectra. The relative amounts of phenolic or photo-Fries products are greatly influenced by the wavelength of light used for the photo-degradation. The use of short wavelength UV light (

Differential Ultraviolet Spectroscopy as an Aid in ... - ACS Publications

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