Chapter 12
Permanence of U V Absorbers in Plastics and Coatings James E. Pickett Corporate Research and Development, General Electric Company, 1 Research Circle, Schenectady, NY 12301
ABSTRACT Ultraviolet absorbers (UVA's) undergo photo-degradation with quantum yields on the order of 10 even in glassy, unreactive matrices such as poly(methyl methacrylate) (PMMA). The kinetics of UVA loss in a film or coating can be described by the equation: A -6
(A0-kt)
= log [(1- T )10 + 1] where A is the absorbance at time t, T is the initial transmission, A is the initial absorbance, and k is a zero order rate constant. The rate constant can be found as the slope of log (10 - 1) plotted vs. exposure. The derivation and implications of these equations and a general review of UVA degradation chemistry are discussed, including the effects of the matrix material, concentration, and hindered amine light stabilizers. The rate of UV absorber loss is of relatively minor consequence when the absorber is used as a bulk additive in a polymer, but is of critical importance in the service lifetime of coatings. The kinetics of photochemical UV absorber loss and use of loss rates in coating lifetime predictions are discussed in detail. 10
0
0
0
A
10
T
Adaptedfroma paper presented at the 7 Annual ESD Advanced Coatings Technology Conference in Detroit, MI Sept. 28-29, 1998. Used with permission from ESD, The Engineering Society.
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© 2002 American Chemical Society
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The photostability of UV absorbers (UVA's) recently has been recognized as key to the lifetime of coatings [1,2,3,4]. The types of molecules typically used as UVA's are shown in Figure 1. These compounds are extraordinarily photostable considering that they are organic molecules. However, the pioneers of UV stabilizers recognized that they are not truly permanent. In 1961, Hirt, Searle, and Schmitt wrote: The protective absorbers are not everlasting; they do photodecompose, but at a much slower rate than the materials which they are designed to protect. The photo-decomposition was found to be dependent on a number of factors including the substrate in which the absorber is dispersed and the wavelength of irradiation [5], Unfortunately, little subsequent work was published to flesh out and substantiate this insight, and the photostability issues of UVA's disappeared from the common wisdom. Questions about coating failure mechanisms arose in the late 1980's and early 1990's that prompted reexamination of UVA lifetimes. We and other groups have been investigating the photostability of UVA's to better understand how UVA's degrade, the kinetics of the process, and how this knowledge allows better prediction of coating lifetimes [6].
1 benzophenone
Ar
2 benzotriazole
H-O
4
3 triazine
oxanilide
Ο 5 cyanoacryiate
Figure 1. Structures of commonly-used commercial UVA's.
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Photophysics of UV Absorbers UVA's are added to coatings and plastics to absorb ultraviolet radiation in the wavelength range of 290 to 400 nm in order to protect materials from weathering. Usually the absorbers have little absorption at wavelengths >380 nm to avoid imparting a yellow color to the products. A key feature of absorbers 1-4 shown in Figure 1 is the presence of a strong intramolecular hydrogen bond between an O-H or N-H group and an oxygen or nitrogen 4 or 5 atoms away. This hydrogen bond allows the energy absorbed by the molecule to be dissipated harmlessly as heat as shown in Figures 2 and 3. The ground state of the UVA is excited to its first singlet state upon absorption of a photon but rapidly undergoes Excited State Intramolecular Proton Transfer (ESIPT) to form the excited state of a tautomer (S'i) which has a smaller energy gap with its ground state (S' ) than the "normal form" tautomer. The small energy gap between S\ and S' makes radiationless decay more facile, and the excited state energy rapidly is lost as vibrational energy to the matrix. The proton then is transferred back to its original position in a very rapid keto-enol tautomerization step. Anything that disrupts the ESIPT process will result in a longer lifetime for the Si excited state and a greater chance of the molecule undergoing irreversible photochemistry. Highly polar matrices can lead to more mtermolecular hydrogen bonding and poorer photostability. In addition, media that are basic could result in deprotonation of a small equilibrium concentration of UVA molecules. Even if the concentration were very low, this small population would be highly susceptible to photolysis and cause rapid loss of absorber. The photochemistry of UVA's is difficult to follow because the rate of degradation is very slow, and many of the primary photoproducts are much less photostable than the starting molecule. Therefore, the concentration of primary products is very small, and only highly degraded fragments usually are recovered. Benzophenones have yielded benzoic acid [5,7] and benzotriazoles have yielded benzotriazole [8,9,10] as shown in Figure 4. The phenolic moieties that would result from photolysis reactions such as these would be very unstable and undergo rapid photo-oxidation. Studies of the photolysis of both benzophenones and benzotriazoles generally show that free radicals alone have little effect on the stability. However, the combination of UV radiation, radicals, and oxygen results in rapid degradation, although the actual mechanisms involved remain unclear [8,10,11]. It should be noted that these photoproducts have very little absorbance in the wavelength range of 290 to 400 nm and cannot function as UVA's. Thus, the absorbance is observed to disappear cleanly as the UVA photodegrades. The photophysics and photochemistry of the cyanoacryiate class (Structure 5) is less well understood. The excited state probably involves charge separation and weakening of the double bond as shown in Figure 5. The weakened double bond would allow enhanced vibration and rapid energy dissipation. The double bond is also susceptible to radical or nucleophilic addition that would result in loss of the chromophore. 0
0
253
hv .R
-Δ
Θ
Η-0
Ν
hv \
R
Ar
R
Ar,
HO
hv
H O,
\
N
-OR
N Ar'
Ar
H
O
H
.Ar Ar,'
O
w
hv 2
I
Ο
H
H
Figure 2. Proton-transfer process in protic UVA 's.
Excited state
$ç~τ ESIPT
S'i Radiationless decay
hv
Proton transfer Ground state
S'o
So " "normal form"
"proton-transferred form"
Figure 3. Schematic of energy dissipation mechanism for protic UVA's.
254
Figure 5. Possible energy dissipation process for cyanoacryiate UVA's.
UVA's, of course, have very low quantum yields for decomposition. A number of studies of various absorbers in various media show quantum yields on the order of 10" to 10" with the lowest values found in very non-polar solvents [10,12,13,14,15]. UVA's in most polymer matrices have quantum yields on the order of 10" . Very polar solvents can increase the quantum yield to 10" . Two studies [1,13] have shown pronounced wavelength dependence for both benzotriazoles and benzophenones with light of 300 nm having quantum yields 3-20x higher than light of 350 nm. 5
7
6
4
Kinetics of UV Absorber Photodegradation The kinetics of UVA loss have been described in detail in several papers [1,3,16]. In this analysis, we will consider the UV absorber in a film or coating and the effect of U V A loss on the total absorbance or transmission of the film. The rate of absorbance loss can described by Equation 1 where k is a rate constant and Τ is the fraction of
255
light transmitted by the film (T = I/I ). Note that the rate is proportional to the fraction of light absorbed (1-T) and not the absorbance (A = -log[T]). The rate constant incorporates the quantum yield for degradation and the incident light flux integrated over all wavelengths. Substituting the relation Τ = 10~ and integrating gives absorbance as a function of time as shown by Equations 2 or 3 where A and T are the initial absorbance and transmission, & is a rate constant, and t is time (or exposure). The rate constant, can be determined by the method of Iyengar and Schellenberg [17] in which Equation 3 is rearranged to give Equation 4. Thus, plotting log(10 -l) as a function of time or exposure should give a straight line with slope k and an intercept related to the initial absorbance. This is demonstrated in Figures 6 and 7. Figure 6 shows absorbance loss data for a PMMA film containing 2% of a benzophenone UVA, Cyasorb® 531, plotted as a function of exposure in a xenon arc Weather-ometer®. Curvature is evident in the data, and a line calculated from Equation 3 using a rate constant of 0.18 A/1000 kJm" fits the data nicely. Figure 7 shows the same data plotted according to Equation 4. The data show an excellent fit to this linear relationship. 0
A
0
0
A
2
àAlât = -k{\~T)
(Eqn. 1) (
4=log [(l-r )l0 ^+l] 1 0
(4
Λ =log (l0 »10
(
0
fc)
-HT* +l)
log(10 -1) = -kt + log(l - Γ ) + A
.2)
(Eqn. 3)
A
0
E q n
0
(Eqn. 4)
In a highly absorbing coating or film, the U V A acts as an inner filter so that the molecules near the surface absorb more photons and degrade more rapidly than those that are deeper into the film. Consequences of this are shown in Figures 8 and 9. When the absorbance of a coating is >1, essentially all of the incident UV photons are absorbed and the loss of absorbance appears linear with time; that is, it shows zero order kinetics. The rate of loss is dependent on only the light exposure. This type of loss is typical of a coating as a whole. When the absorbance is 1 can be described by A = A - kt. t
0
50
100
150 200 Time (Arbitrary Units)
250
300
0
350
Figure 9. Calculated UVA loss for a highly absorbing coating plottedfirstorder kinetics. The absorbance in the range A < 0.1 can be described by the equation log(A ) = log(A )-kt. t
0
258 We have observed both kinds of kinetic behavior when measuring the loss of UVA from a silicone hardcoat containing a benzophenone-type UVA during outdoor weathering [1]. Figure 10 shows the loss of absorbance from the entire coating applied to polymethyl methacrylate (PMMA) and exposed outdoors. The sample exposed in Mt. Vernon, IN showed approximately linear loss of absorbance, as predicted. A series of samples tested in Florida of the same coating applied to polycarbonate were tested by attenuated total reflectance IR which analyzes only the very surface of the sample; most of the information comes from the top few tenths of a micron where the absorbance due to the UVA is < 0.1. The loss could be seen to obey first order kinetics as shown in Figure 11. Descriptions of UVA lifetime in a material can be ambiguous [16]. The concept of a half-life is applicable only to the region of absorbance
