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C. D. MILLER Fabrics and Finishes Department, Experiment Station, E. I. du Pont de Nemours & Wilmington, Del.
Kinetics and Mechanism of
Suitability of paints for outdoor use depends on extent of deterioration, yet methods for measuring this reaction have been largely empirical. This inconsistency may be eliminated by the simple accurate method described here, which measures, by changes in visible reflection spectra, alkyd deterioration which depends on wave length and intensity of light, film thickness, and originates in dryingoil modifiers
IN
PREVIOUS studies of alkyd degradation (7, 2 ) the effect of ultraviolet irradiation in producing volatile decomposition products, gloss loss, thickness changes, and other phenomena was observed; inferences as to the reaction mechanism were made. In the present work, more precise optical techniques have been employed to permit a more accurate kinetic study of the photodegradation process. The reaction can now be described quantitatively as a function of intensity and wave length of light and of the film thickness of the sample. The classical techniques for studying polymer degradation are ill suited to the investigation of a typical crosslinked alkyd film. O n the other hand, the conventional methods for observing degradation in paint films are essentially empirical and qualitative, frequently lacking the accuracy and precision required for a kinetic study. Yet these manifestations of degradation, notably
Co., Inc.,
Alkyd Photooxidation
gloss loss, are the factors which largely determine the suitability of a particular coating for exterior uses. This loss of gloss has been shown to occur as the top layer of clear polymer is eroded away leaving pigment particles on the surface. I t is not unreasonable, therefore, that there is a simple relationship between the rate of gloss loss of a pigmented film and the erosion rate of a very thin layer of clear polymer. If the thickness of the film is of the order of the wave length of light, these thicknesses, and, more important, the changes in thickness which accompany irradiation by ultraviolet light, can be observed interferometrically with high precision. In this study, clear films of approximately 1 micron were irradiated, and the thickness changes were measured by observing the changes in the interference bands in the visible reflection spectra. This proved to be a simple, nondestructive, and accurate (to 0.1%) method for studying alkyd degradation.
Experimental
The polymer used in this work was a linseed-tung oil alkyd, 50% oil length, acid number 18, without the addition of any drier. Thin films of this alkyd were deposited on chromium-plated microscope slides from an approximately 30% solution in a volatile solvent using the apparatus shown. A detailed description of this technique has been published ( 5 ) . Films were dried in air a t 120' C. for 1 hour. Interference spectra were obtained in the visible region with a General Electric recording spectrophotometer. Thicknesses were calculated from the
reflection curves according to the usual formulas 2m Thickness = - A,, 4n
where m is the order of the interference. For purposes of calculation, n, the refractive index, of the films was assumed constant a t 1.5. Values so obtained were reproducible to 0.1%. Films for infrared measurements were spread on rock salt plates in the conventional manner and dried as indicated above. Infrared spectra were obtained with a Perkin-Elmer Model 21 spectrometer. The ultraviolet spectrum of the film was obtained with a Beckman DU spectrophotometer. For this purpose, films were deposited on half chromiumplated silica slides so that the absorption might be measured by transmittance and the thickness of reflection. Ultraviolet exposures were made in air under a Hanovia 125-watt, medium pressure, quartz-jacketed mercury arc. Samples were placed on a small turntable equipped with stages a t two distances from the lamp to give two intensities. The ratio of the light intensities was 3 to 1. Different wave length distributions were produced with standard Corning transmittance filters, 9700, 7740, and 7380, all 2 mm. thick, whose transmittance characteristics have been given (7). The wave length distributione so obtained are hereafter described as >2000 A. (unfiltered), >2500 A., >2950 A., and >3500 A. Discussion of Results
Interference Data. The rate of erosion is approximately proportional to VOL. 50, NO. 1 e
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Dipping apparatus used for depositing thin alkyd films on chromium-plated microscope slides
STAND\
Films were deposited on half chromium-plated slides so that absorption could be measured by transmittance and the thickness by reflection
TRANSMITTED
the light intensity, and this relationship is reasonably independent of wave lengrh, in agreement with earlier data on gloss loss and rate of evolution of volatile products (7). Table I gives the ratio of exposure hours required to reach the same percentage film loss for samples of the same initial thickness at the two light intensities. The effect of wave length on the rate of erosion was measured for samples of 10,000 A. initial thickness. Table I1 shows that the initial over-all quantum yield is almost constant at the various
wave lengths, as calculated from the radiant energy of the lamp, the transmittance of the filters, and the absorption spectrum (Figure 1) of the polymer. The somewhat higher quantum yield observed for the reaction at h >3500 A. probably reflects the greater relative contribution of purely thermal oxidation to the observed rate. The lack of dependence of the quantum yield on wave length is in good agreement with other data on similar polymers (2, 4). As shown below, the mechanism of the reaction at all wave lengths >2500 A. is
Dependence of Rate of Film Loss on Intensity of Irradiation
Table 1.
Wave Length, A.
Film Remaining, %
>2000
90 80
Exposure Time, Hours At 11 At 1x1
TiinedTimexr
60
20 39 59 77
7 14 21 27
2.9 2.8 2.8 2.9
> 2500
95 92 88
25 63 142
7 19 44
3.5 3.3 3.2
>2950
98 96 94 92
22 54 100 148
8 20 40 52
2.8 2.7 2.5 2.8
> 3500
99 98 97
36 81 130
16 35 56
2.2 2.3 2.3
70
Table II.
the same, and therefore it is not surprising that the quantum yield is constant in this interval. The similarity of the quantum yield at lower wave lengths, a t which the reaction is distinctly different, was somewhat less expected. When films of different initial thicknesses were exposed to the various wave length distributions, the erosion produced by very short wave-length radiation was a true surface reaction, essentially independent of thickness. Longer wave lengths produced a loss of film thickness which was approximately proportional to the original thickness, suggesting a bulk reaction. This is quantitative confirmation of the qualitative inferences drawn earlier ( 2 ) of the nature of the reaction at various wave lengths from the carbon-hydrogen ratios of the volatile decomposition products. This is consistent with the ultraviolet spectrum of the polymer; short wave lengths are so strongly absorbed that the interior layers are effectively shielded: Typical data are given in Figure 2 for unfiltered light and in Figure 3 for longer wave
Effect of Wave Length on Rate of Film Loss
Wave Length, A.
Time t o Erode 550 A./Sq. Cm., Hr.
>2000 > 2500 > 2950 > 3500
3.5 10 37 165
Energy absorbed, Einstein/Hr. 8s. Cm.
x
10-6
1.577 0.625 0.150 0.023
Quantum Yield, A./
Einstein X 10' 1.00 0.88 1.00 1.45
I I . _ , L_ 2000 2500 3000 3500 4000 4500 WAVE LENGTH IN ANGSTROMS
Figure 1 , Ultraviolet spectrum of alkyd film Film thickness, 1 micron
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INDUSTRIAL AND ENGINEERING CHEMISTRY
ALKYD P HOTO-OXIDATION
- REL INTENSTY =3
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--.-REL. INTENSTY = I 6ooo
-WAVE LENGTH~ 2 9 5 0A.. WAVE LENGTH 7 2500 A.. ORIGINALTHICKNESS 0 5 ~ A ORIGINALTHICKNESS LOW
c3
z -
ORlG THICKNESS=05u A 0RIG.THICKNESS- I Ou
f
a
10
2 0 3 0 4 0 5 0 6 0 HOURS-
Figure 2. Rate filtered light
Figure 3. Rate of film loss at various wave lengths
of film loss under un-
lengths. The extent of degradation at X >3500 A. was too slight to be considered significant. Kinetically, the reaction under unfiltered light was zero order with respect to thickness with rate constants of 133 A. and 48 A. per hour, respectively, a t the two light intensities. At longer wave lengths, the dependence of film loss o n sample thickness suggests a first-order type of reaction; however, calculation according to this scheme does not yield a satisfactory rate constant. A typical calculation is given in Table I11 for a
Table 111. Calculation of First-Order Rate Constant for Film Loss at X
Time, Hours
>2950 A. Film Thickness Remaining, A.
k X 10*
0 6 22 44 63 108 128 148 165
9160 9000 8760 8580 8450 8130 8050 7970 7920
2.66 1.96 1.46 1.28 1.13 1.11 1.10 0.88
'
sample of initial thickness 9160 A. irradiated with light of X >2950 A. Similar results were obtained for all samples irradiated with light of X >2500 A. This is interpreted as indicating, at these wave lengths, the selective oxidation of the more reactive groups, leaving a film of progressively increasing stability toward further degradation. At short wave lengths, the reaction has been shown above to occur largely on the surface, and therefore no such stabilization of the bulk of the polymer is to be expected. Infrared Spectroscopic Data. From the rate measurements it was suggested that the chemical nature of the films was changing during exposure; and to characterize the degradation process fully, it was necessary to observe these chemical changes. Conventional chemical analyses were not considered suitable, as they consume gross amounts of sample and have doubtful accuracy for crosslinked polymers. Infrared spectroscopy, on the other hand, is a rapid, accurate, and nondestructive alternative, permitting a single sample to be followed during the entire course of the degradation. The spectrum of the polymer is shown
in Figure 4. The spectrum has been discussed in detail many times before; therefore, consideration is limited to the 2.8-micron hydroxyl stretching, the 3.4-micron aliphatic C-H stretching, the 5.8-micron carbonyl stretching, and the 13.5-micron out-of-plane C-H bending absorption of the o-disubstituted aromatic nucleus. By means of these, it was possible to follow the course of the reaction. By way of illustration, Figure 4 shows the typical changes in the spectrum during photodegradation a t X >2950 A. and relative intensity = 3. Qualitatively, there was a broadening of the hydroxyl and carbonyl bands, undoubtedly resulting from the formation of complex oxygenated products, and an increase in the general absorption from 8 to 10 microns, also probably attributable to singly- and doublybonded oxygen. More informative were the intensity changes which accompany the photo-oxidative process. Figures 5, 6, and 7 show the disappearance of functional groups for each wave length distribution, as a function of exposure time. These data were obtained from the peak intensities of the characteristic infrared bands discussed above.
W 0
z a
5.
ts
-8EFORE
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z
AFTER 135 HRS. U V .
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P
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I 6
I 6
Figure 4.
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I 6 a MICRONS I
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Spectrum of drying oil alkyd
s
HOURS
Figure 5. Disappearance of functional groups resulting from exposure to unfiltered light VOL. 50, NO. 1
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201 I
m
m
80
10012014Ci-
bOURS
Figure 6. Disappearance of functional groups resulting from exposure to wave lengths >2500 A.
-67
80
m
140
izo
HOURS
Figure 7. Disappearance of functional groups resulting from exposure to wave lengths >2950 A.
% PHTHALATE DEGRADED ALKYD CEGRACATIW
Figure 8. Effect of wave length on a1kyd degradation
While the translation of these peak intensity measurements into functional group concentrations is obviously only an approximation (particularly for hydroxyl groups), this simple treatment of the data yields interesting results. The steady decrease of aliphatic carbonhydrogen groups with the concomitant increase in hydroxyl and carbonyl must be interpreted as the oxidation of the drying oil hydrogens to hydroxyl, hydroperoxyl, and carbonyl groups. The latter first increase and then decrease as the C-H links become fewer (and more stable), because their rate of decomposition now exceeds their rate of formation, in a manner characteristic of the intermediates in consecutive chemical reactions. Complex carbonyl structures, especially if conjugated-for example, vicinal dicarbonyls -are unstable to ultraviolet radiation and decompose with chain scission to produce volatile products, free radicals, and new olefins and ketones which are subject to repetition of the same process. This interpretation of the oxidation reaction sequence in the drying oil alkyd is in accord with the well known studies of Bolland, Bateman, Farmer, and others. A recent book on polymer degradation contains an excellent summary of this research ( 3 ) . These data show that the aromatic nuclei are more stable than the aliphatic portion of the polymer a t longer wave lengths; but under‘ unfiltered light, where they absorb energy directly, the aromatic nuclei decompose rapidly and even preferentially. As “decomposition” is measured spectroscopically, this refers only to the change of the o-disubstituted aromatic structure to any other chemical species and not necessarily to the loss of phthalate carbon and hydrogen from the film. If the percentage decrease in the 3.4micron band, interpreted as aliphatic C-H, is compared with the percentage decrease in the 13.5-micron aromatic band at the same exposure time for various wave lengths, as shown in Figure 8, several conclusions are suggested. First, the reaction is fundamentally different at very short wave
lengths; this was also observed kinctically in the thickness loss experiments. Second, the reaction is much less specific at short wave lengths; this also is in agreement with thickness loss data. Third, the decreasing slope of both of these curves indicates that the aliphatic portion is becoming progressively more stable to photodegradation. The stabilization of the polymer predicted earlier from rate measurements has now been localized within the molecule. Fourth, a comparison of these two curves suggests that, a t short wave lengths, the phthalate is decomposed directly, while a t longer wave lengths, at which the phthalate is transparent, its decomposition is a secondary process sensitized by the decomposition of some poi tion of the aliphatic groups. The absence of a band a t 3.2 microns in Figure 4, which was present in the unbaked alkyd, indicates that the residual unsaturation in the cross linked film is very low; therefore, it is probable that the site of the oxidative attack was at the tertiary hydrogen of each cross link. Either residual unsaturation or, more probably, secondary structures arising from the double bonds constitute the reactive sites for photo-oxidation, A simultaneous study of the degradation of a comparable stearic acid alkyd showed that it was much more durable toward radiation at X > E O 0 A., not only in the aliphatic portion, which was different, but also in the aromatic portion which was identical with that in the drying oil alkyd. The spectrum of this polymer before and after irradiation is shown in Figure 9. This is in accord with other evidence for the photosensitization mechanism of phthalate decomposition in drying oil alkyd films. Acknowledgment
The author wishes to acknowledge the assistance of John J. Wenke in obtaining the experimental data reported herein. literature Cited (1) FitzGerald, E. B., ASTM Bull. iYo, 207. 65 11955).
(2) FitzGerald,’ E. B . , TND. ENG. CHEM. 45, 2545 (1953). ( 3 ) Grassie, N., “Chemistry of High Polymer Degradation Processes,” Interscience, New York, 1956. (4) Gusman, S.. SDell, A.. Division of Paint,’ Plastiis, and ’Printing Ink Chemistry, 130th Meeting, ACS, .4tlantic City, N. J., September 1956. (5) Xliller, C. D., J . Polymer Sci. 24, 311 (1957). ,
-BEFORE
I
EXPOSURE
.“-,-AFTER 135 HRS. UV.
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RECEIVED for review February 7, 1957 ACCEPTEDJune 27, 1357 2
3
4
0
6
7
B
a
10
I1
MICRONS
Figure 9.
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Spectrum of stearic acid alkyd
INDUSTRIAL AND ENGINEERING CHEMISTRY
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I3
14
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Division of Paint, Plastics, and Printing Ink Chemistry, 129th Meeting, ACS, Dallas, ‘rex., April 1956.
