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Case Western Reserve University, Solar Durability and Lifetime Extension Center and
Materials Science Department, Cleveland, Ohio 44106 roger.french@case.edu
Abstract.
In the development of materials for enhanced photovoltaic (PV) performance, it is critical to have quantitative knowledge of both their initial performance and their performance over the required 25-year warranted lifetime of the PV system. Lifetime and degradation science, based on an environmental stress and response framework, is being developed to link the intensity and net stress to which materials, components, and systems are exposed to the responses observed and their subsequent degradation and damage accumulation over the lifetime. Induced absorbance to dose (IAD), a metric developed for solar radiation durability studies of solar and environmentally exposed materials, is defined as the rate of photodarkening or photobleaching of a material as a function of radiation dose. Quantitative degradation rates like IAD, determined over a wide range of stress intensities and net stresses, have the potential to predict degradation, failure, and power loss rates in photovoltaic systems over time caused by damage accumulation.
Two grades of poly(methyl methacrylate) were exposed and evaluated in two cases of highintensity ultraviolet exposures. A three- to six-fold increase in photodarkening was observed for one acrylic formulation when exposed to UVA-340 light when compared with concentrated xenon-arc exposure. The other, more highly stabilized acrylic formulation, showed up to three times more photodarkening in the same exposure.
© 2012 Society of Photo-Optical Instrumentation
Engineers (SPIE).
[DOI: 10.1117/1.JPE.2.022004
]
Keywords: photodegradation; acrylic; poly(methyl methacrylate); photovoltaics; degradation.
Paper 12020SS received Mar. 21, 2012; revised manuscript received Jul. 26, 2012; accepted for publication Oct. 1, 2012; published online Nov. 19, 2012.
1 Introduction
A recent U.S. Department of Energy workshop on Science for Energy Technologies
1
topic of photovoltaics (PV) lifetime and degradation science (L&DS)
2
–
4
identified the as a critical scientific challenge for robust adoption of PV. The 25-year lifetime performance of PV requires a better understanding of the degradation mechanisms in PV materials, components, and systems. By developing metrics, metrology, and tools to quantify, compare, and cross-correlate the response of PV systems, components, and materials to a variety of stressors such as ultraviolet (UV) radiation, humidity, and temperature variation for both accelerated and outdoor testing, we can link stresses to observed responses in a stress-response framework and determine quantitative rates of degradation. L&DS requires the development of quantitative degradation mechanisms and rates for degradation and failure modes, so as to enable quantitative lifetime projections.
Lifetime and degradation science (L&DS), based on a stress and response [ R
ð
S
Þ
] framework, is being developed that links the intensity and net stress to which materials, components, and systems are exposed to the responses observed and the degradation and damage accumulation over lifetime. This R
ð
S
Þ framework can encompass multifactor, and cyclic, environmental stressors including solar irradiance, temperature, and humidity, which can cause degradation over time. The R
ð
S
Þ framework allows for the cross-correlation of the materials response and degradation rates with numerous applied stress intensities and net stresses and allows determination of the effects of stressors, even at accelerated intensities with applicability to multiple service conditions. At the same time, an R
ð
S
Þ framework permits incorporation of data determined over
0091-3286/2012/$25.00 © 2012 SPIE
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Murray, Bruckman, and French: Photodegradation in a stress and response framework . . .
Fig. 1 Reliability engineering curve showing the three main statistical regimes: infant mortality, random failures, and failure at the end of life.
many stress conditions and corresponding to many different responses and its cross-correlation to produce a more complete picture of a material
’ s or system
’ s stress and response when compared to the traditional acceleration factors approach, which is always in reference to one set of service or application conditions. The R
ð
S
Þ framework requires the use of quantitative metrics to be used such as induced absorbance to dose (IAD) which is defined as the rate of photodarkening or photobleaching of a material as a function of radiation dose.
5 , 6
In the R
ð
S
Þ framework, all of the available responses of a material are integrated to provide a comprehensive scientific understanding of the material.
4
,
7
Reliability engineering is thought to fall into three main statistical regimes: infant mortality, random failures, and failure at the end of life
8
(Fig.
1 ). Initial performance qualification testing,
represented by UL 1703,
9
IEC 62108,
10
IEC 61215,
11
and IEC 61646,
12
allows for reasonable confidence intervals when mitigating infant mortality and can sometimes be useful in predicting random failures in PV modules, which are failures that occur imminently or randomly after deployment due to process and manufacturing defects.
13 , 14
Lifetime performance, however, requires knowledge of the power degradation rate of a PV system or module, a quantity that is currently only quantified after deployment has already occurred. It is thought that by performing highly accelerated stress tests, degradation modes of materials can be elucidated and with confirmation from outdoor testing, lifetime performance can be predicted.
Mirror augmented photovoltaic systems utilize mirrors to couple more light onto a PV module and its absorber. By increasing the light absorbed, they have the potential to provide a lower levelized cost of electricity (LCOE).
15
Low concentration photovoltaic (LCPV) systems, where solar irradiance is concentrated by a factor of 1 to 10, present opportunities for cost benefits when compared with traditional technologies. In these systems, electrical output per unit area of active materials increases nearly linearly with concentration factor, thereby reducing the cost of active materials per watt by up to a factor of 10, while still having the opportunity to utilize traditional 1 sun PV technologies. At the same time, increased solar irradiance from LCPV amplifies the stress intensity and possibly the degradation rates of the systems, components, and materials.
Additionally there is extensive interest in high concentrating photovoltaics applications for use of Fresnel lenses fabricated from durable poly(methyl methacrylate) (PMMA) grades.
16
Polymers of many types are vulnerable to UV degradation because the highest energy UV radiation observed in the solar spectrum has energies that surpass all but the strongest carbon bonds. In order to protect these polymers, various formulations of UV absorbers, hindered amine light stabilizers (HALs), and radical scavengers have been employed to stabilize materials
exposed to UV degradation.
17
A common class of UV absorbers are hydroxyphenyl benzotriazole compounds such as Tinuvin
® between 0.1 and 1 wt. %
20
made by BASF,
18 , 19
which are usually used at concentrations and protect the polymer matrix by being sacrificially degraded by the
UV light.
18
The degradation of numerous grades and formulations of PMMA, acrylic polymers, has been studied under a wide variety of UV light sources, including mercury vapor lights emitting at 253.7 nm,
21 – 25 xenon arc lamps filtered to match AM 1.5,
26
and
60
Co emitted gamma
radiation.
21
Additionally, studies of the wavelength dependence of the photodegradation
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Murray, Bruckman, and French: Photodegradation in a stress and response framework . . .
have been performed using glass filters,
26
emitting at 300 nm and 350 nm.
22
monochromators,
27
,
28
and with different light sources
Indoor exposures have been performed under vacuum and in air, with mass loss metrology and gaseous degradation products being identified by mass spectroscopy.
22 , 24 , 25
Production of degradation products and viscosity averaged molecular weight distributions have been used to determine the quantum efficiency of UV light in degrading
PMMA.
21
,
24 , 25 , 27
,
29
Others have performed outdoor exposures with flat plate exposures,
26
,
30
,
31 and with accelerated tracking exposures.
27
Optical characterization of degradation in acrylics has been done using colorimetry,
26 , 31
cal density,
28 , 29
UV spectroscopy,
16 , 24
–
30
,
32
–
35
electron spin resonance spectroscopy,
21
optiand Fourier transform infrared spectroscopy.
16
,
28
,
30 , 32
,
35
using atomic force microscopy,
16
Surface degradation of acrylics has been studied scanning electron microscopy,
30
and contact angle measurements.
16
Mechanical testing has been used to characterize the effects of UV degradation on
PMMA mechanical properties.
31
,
34
Other studies have been done to analyze the thermal decomposition and depolymerization of PMMA associated with mass loss and increased photodegradation.
36
Degradation of PMMA by ultraviolet light is thought to occur when high energy incident radiation initiates radicalization of an ester side group leading to
β
-scission of the polymer backbone and decomposition of the radical end to products including H
2 and HCOOCH
3
.
25
The loss of the ester group creates a greater free volume in the PMMA,
increasing likelihood of creep and dimensional instability.
37
, CO, CO
2
, CH
2
OH
Chain scission that occurs from exposure to solar radiation is predominantly caused by UV radiation with wavelengths shorter than 320 nm.
27 , 28
This process is accompanied by an increase in yellowing in many experiments.
The color centers responsible are identified as free radicals trapped with the bulk material until they become neutralized by oxygen or other radiation.
38
Typically, UV exposure of materials can be performed with a xenon arc light source, which can be filtered to closely match the shape of the solar spectrum for air mass (AM) 1.5, a standard representing real-world exposure at 48.2 deg latitude.
39
AM 1.5 exposures can be performed at multiple levels of irradiance by using concentrating optics to focus the light beam. This enables proper weathering acceleration because spectral matching between outdoor conditions and indoor exposures excludes degradation induced by radiation outside of typical environmental conditions. However, due to their lower operating costs, fluorescent exposure of materials using
UVB-313, UVA-340, or UVA-351 lamps are often used as a less expensive way to provide insights into material performance over time. UVA-340 lamps radiate light that closely matches the AM 1.5 spectra in the region from 280 to 360 nm when the spectra radiance peaks at 340 nm with a value of 0.30 W
∕ m 2 ∕ nm .
As a case study to demonstrate the L&DS approach to lifetime and degradation science, we report here a study on photodegradation in two acrylic PMMA grades under two different irradiation sources. A Q-Lab QUV accelerated weathering tester
40
outfitted with UVA-340 lamps was used to expose samples to UV radiation; this method for exposure was compared with a
Newport solar simulator equipped with a concentrator. The ASTM G154 Cycle 4 without the condensation step QUV exposure was performed with spectral irradiance peaking at
1.55 W
∕ m 2 ∕ nm at 340 nm, while the xenon arc exposure was performed with full spectral irradiance (integrated over the AM 1.5 spectrum) controlled at 50.4 kW
∕ m 2 . The temperature inside the QUV tester was kept at 70°C with a sample temperature of approximately 67°C, while temperature inside the Newport enclosure was uncontrolled, producing a typical sample temperature of approximately 50°C.
41
Different formulations of PMMA were exposed to these single-factor accelerated exposures. Average spectral IADs and yellowing indices (YI) for the exposures were determined and correlations between exposures are discussed.
2 Stress-Response [
R S
] Framework for Solar Degradation
Stressors that impact PV materials and components can be characterized in terms of instantaneous stress level (
σ
), and net stress or integrated stress ( S ), which is the instantaneous stress level integrated over the length of time the stress was applied [Eq. (
1
)]. Changing the instantaneous stress level may change the material
’ s response characteristics; therefore, stressors must be quantified in terms of both instantaneous stress level and integrated stress [Eq. (
2
)]:
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Murray, Bruckman, and French: Photodegradation in a stress and response framework . . .
S
¼
Z
σ d t
X
S
¼
Z
ð σ i
⊗ σ j
: : :
⊗ σ n
Þ d t;
(1)
(2) where
σ i and
σ j are different stress types and levels and
⊗ represents the convolution of those stresses.
A material
’ s response ( R ) to both instantaneous stress level and integrated stress may correspond to a change in the optical properties of a material, a loss of mechanical strength, or any measurable change in properties arising due to stressors applied over time. The general
relationship between stress and response is a function of that stress [Eq. ( 3 )]:
R
¼ f
ð σ
S
Þ ¼
Z f
ð σ Þ σ d t: (3)
Single factor accelerated testing is often performed on materials to determine their durability to a particular environmental stress. By varying the stress intensity, it is possible to show the range of conditions that do not open new degradation pathways. An expansion of this methodology to multiple stresses gives [Eq. (
4
)]:
Z
R
ð
S i
; S j
; : : : S n
Þ ¼ f
ð σ i
;
σ j
; : : :
σ n
Þð σ i
⊗ σ j
: : :
⊗ σ n
Þ
; (4) where S i
; S j
; : : : S n are different stresses that cause responses. The benefit of such a framework is that stresses can be separated and accounted for in this methodology. However, single stress exposures introduce hazardous assumptions because some stresses have synergistic effects, meaning that a correlation function between stress and response in real-world environments can depend on all stresses applied to the system. A multifactor test, which includes multiple stressors, can lead to a better understanding of the synergistic effects of stress in a realworld environment. Response is therefore a function of the convolution of multiple stressors at their service-use conditions. The convolution of these stresses implies that they can have synergistic effects. Therefore, the correlation function, which describes the impact of multiple stressors in environmental conditions, while unknown, depends on all stresses applied to the system. If an accelerated exposure can be shown to induce the same response as observed in environmental conditions, a correlation function can be used to predict the response observed in a system exposed to environmental conditions without waiting for real-time testing results.
3 Methods
3.1
Two formulations of PMMA used in the present study are multipurpose (MP) acrylic and UV transparent (UVT) acrylic samples. Each contains different amounts of the same UV stabilizer.
These acrylic samples were from Replex, Plastics, Inc.
42
MP grade acrylic is used in the security and transportation industry as a substrate for mirrors and housings, while UVT is used for daylighting
43
applications that require full spectrum light. The thicknesses of the acrylic samples were approximately 3 mm.
3.2
Exposures were performed with a Q-Labs QUV Accelerated Weathering Tester (Model QUV/
Spray with Solar Eye Irradiance Control)
40
and with a Newport 1.6 kW diverging beam solar simulator (Model 92190) with a 13
× irradiance concentrator (Model SP81030-DIV). The two different stress conditions are compared in Table
1
. By sequential measurements of optical
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Murray, Bruckman, and French: Photodegradation in a stress and response framework . . .
Table 1 A comparison of the stress conditions.
Stressors
Total irradiance Full spectrum 280 to 4000 nm
UVA-340 280 to 360 nm
TUV 280 to 400 nm
Dose step
Total dose Full spectrum 280 to 4000 nm
UVA-340 280 to 360 nm
TUV 280 to 400 nm
Time (days)
Sample temperature (°C)
Newport solar simulator (50
×
)
50
.
4 kW
∕ m
2
576
W
∕ m
2
1570
W
∕ m
2
13
.
06
GJ
∕ m
2
AM1.5
78 .
5 GJ ∕ m
2
1 .
51 GJ ∕ m
2
2
.
44
GJ
∕ m
2
18
50
QUV (
1
.
55
W
∕ m
2 at 340 nm)
∕ nm
84
.
5
W
∕ m
2
60
.
65
W
∕ m
2
84
.
5
W
∕ m
2
62 and
48
MJ
∕ m
2
UVA-340
287 MJ ∕ m
2
206 MJ ∕ m
2
287
MJ
∕ m
2
39
67 properties after different steps of exposure dose (or exposure doses), it is possible to determine the relationship between solar irradiance (the stress) and induced degradation rates of optical properties (the response) of these materials, along with the form of the dose dependence of the response (linear, exponential, sub-, or supra-linear). This 18-day exposure is equivalent to radiant exposure of 8.6 years of tracked exposure in Phoenix, Arizona.
44
The QUV UVA-340 exposures had eight of each type of acrylic samples so as to provide sufficient statistics. A representative sample was removed after each dose step and the baseline sample was never exposed. The first dose step was 284 h, and each subsequent dose step was
220 h. There was only one sample for each type of acrylic exposed in the Newport solar simulator at 50.4 kW
∕ m 2 . Each dose step was 72 h. The baseline abs/cm and YI measurements were performed on the acrylic samples prior to exposures.
3.2.1
The QUV accelerated weathering tester uses customized fluorescent lamps to expose samples to high doses of UV radiation, which simulates different types of damaging environmental stress conditions. The exposures were performed with UVA-340 lamps, which emit radiation between
280 and 400 nm. This closely matches the AM 1.5 spectrum at wavelengths shorter than 360 nm, where much of the damaging radiation exists in the solar spectrum. The QUV was run using
ASTM G154 Cycle 4 (
21 days.
41
1.55 W
∕ m 2 ∕ nm at 340 nm at 70°C) without the condensation step for
By setting the QUV peak intensity to 1.55 W
∕ m 2 ∕ nm at 340 nm, a higher level of damage can be induced, making this a single-factor accelerated test compared to outdoor weathering. This level of UV radiation is 5.2 times higher than the intensity of AM 1.5 at 340 nm.
While the integration of the QUV spectra gives a much lower total dose, the ratio of light in the damaging range from 280 to 360 nm is much higher. Only 1.92% of AM 1.5 radiation falls into the UVA-340 exposure wavelengths. By correcting the TMY3 data for this factor, the 944 h exposure is equivalent to 1.2 years of tracked outdoor exposure in Arizona.
44
This was calculated with Eqs. (
5
) and ( 6 ), where
E e λ length in nm, E e
280 nm and 360 nm ( J
∕ m 2 ).
45
is spectral irradiance for each source ( W
∕ m 2 is irradiance ( W
∕ m
2
) and H e
∕ nm ),
λ is waveis radiant exposure to wavelengths between
E e
¼
Z
360
E e
λ d
λ
280
(5)
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Murray, Bruckman, and French: Photodegradation in a stress and response framework . . .
H e
¼
Z
E e d t
¼
Z
0 t
Z
360
E e
λ d
λ
:
280
(6)
3.2.2
The Newport xenon arc solar simulator
46
was used to irradiate material samples with spectral matched AM 1.5 radiation
39
using a Newport 13x high flux concentrator accessory with the solar simulator, irradiance levels of 50.4 kW
∕ m 2 were achieved, making this a single-factor accelerated test compared to both UVA-340 and outdoor exposure. Continuous power monitoring with a
Newport power meter (Model 1918-R) was used to monitor the irradiance, making it possible to quantify the irradiance and dose to which a sample was exposed. Integrated dose was calculated for total dose and UVA-340 [Eqs. (
5
) and (
6
)]. Compared with outdoor exposures in Arizona, using typical metrological year data (TMY3) supplied by NREL,
44
which are about 9.1 GJ
∕ m 2 per year, this 18-day test exposed samples to doses equivalent to approximately 8.5 years.
3.3
3.3.1
An Agilent Cary 6000i spectrometer with a diffuse reflectance accessory-1800 (DRA) and an
InGaAs detector was used to determine the Abs/cm for each acrylic sample. The spectra were acquired from 180 to 1800 nm every 0.40 nm with a scan rate of 48.00 nm
∕ min and a spectral bandwidth of 2.00 nm in the UV-Vis and 4.00 nm in the NIR regions.
3.3.2
The HunterLab UltraScan Pro was used to determine the YI of each of the acrylic samples. This instrument is a fast, high-performance color measurement spectrophotometer with a spectral range from 350 to 1050 nm with a 5 nm optical resolution.
3.3.3
The temperature for the samples in the Newport solar simulator and the QUV accelerated weathering tester were determined with a FLIR infrared camera (Model T300).
47
3.4
3.4.1
Induced absorbance to dose (IAD) is a measurement of the change, at a particular wavelength, of the bulk optical absorbance per centimeter, base ten (Abs/cm) of a material, and this analysis assumes uniform bulk absorption phenomena, which may not be observed in certain samples.
Therefore IAD analysis is assumed to be independent of thickness. In these samples, preliminary evidence shows that photodegradation progresses from the light-exposed surface through the bulk of the material. Therefore, the IAD metric as used here represents the average of the optical absorbance per centimeter through the thickness of the sample. IAD is a quantitative dose metric for photodarkening and photobleaching on a per unit dose basis.
48 , 49
The per unit dose basis can be changed so that IAD can be calculated per full spectrum dose or per UVA-340 dose. Incremental IAD is useful to track and identify transient phenomena, which tend to stop after a certain dose (e.g. phenomena associated with curing or photobleaching of impurities). While average
IAD is useful to follow over large doses and long exposure times, since the total dose exposed increases, the contribution due to transient phenomena are reduced in amplitude, and continuous degradation processes end up being the predominant observed process in the average IAD.
If either calculation of IAD reports the same values for successive dose steps, then the material is behaving linearly, and if the same values of IAD are seen with two different exposures, then
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Murray, Bruckman, and French: Photodegradation in a stress and response framework . . .
reciprocity in irradiance is being obeyed. The average IAD [Eq. (
7 )] and the incremental IAD
[Eq. ( 8 )] are given by:
Average
Abs per cm
GJ dose
¼ m
2
Abs t
ð λ Þ ∕ cm
−
Abs i
ð λ Þ ∕ cm
Dose t
(7)
Incremental
Abs per cm
GJ m 2 dose
¼
Abs i
þ
1
ð λ Þ ∕ cm
−
Abs i
Dose i
þ
1
−
Dose
ð λ Þ ∕ cm i
: (8)
IAD values were determined with the Agilent Cary 6000i with a DRA-1800.
3.4.2
Yellowness index (YI) as defined in the ASTM E313
50
is a colorimetric measure of yellowing.
Because YI is measured over a broad wavelength range, it is more sensitive than typical spectral measurements. YI is calculated from the transmittance values for
λ ¼
380 to 780 nm.
50
Yellowness indices are also useful because they are closely linked to reduced optical performance.
16
YI can be determined by Eq. ( 9 ):
YI
¼
1000 ð C x
X − C z
Z Þ
Y
; (9) where X , Y , and Z are the CIE tristimulus values. For D 65
∕
10 ° , Cx
¼
1.3013
, Cz
¼
1.1498
.
50
YI was determined using the HunterLab UltrascanPro colorimeter. Yellowing rates, the rate of change in yellowness as a function of dose, were determined by Eq. (
10 )
R
YI
¼ d
ð
YI
Þ d
ð
S
Þ
: (10)
4 Results
The absorbance per centimeter, base 10 results of two different types of exposures on two formulations of acrylic PMMA, UVT and MP, are reported in Figs.
2
and
3
, respectively. Baseline spectra of both formulations of acrylic show a fundamental absorption edge near 275 nm
Fig. 2 (a) Abs/cm for ultraviolet transparent (UVT) grade poly(methyl methacrylate) (PMMA) for exposures in the QUV accelerated weathering tester to UVA-340 irradiance for baseline, one to four dose samples. (b) Abs/cm for UVT grade PMMA for exposures in the Newport solar simulator to AM 1.5 irradiance for baseline, one to six dose samples.
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Murray, Bruckman, and French: Photodegradation in a stress and response framework . . .
Fig. 3 (a) Abs/cm for multipurpose (MP) grade poly(methyl methacrylate) (PMMA) for exposures in the QUV accelerated weathering tester to UVA-340 irradiance for baseline, one to four dose samples. (b) Abs/cm for MP grade PMMA for exposures in the Newport solar simulator to AM 1.5
irradiance for baseline, one to six dose sample.
(region 1); however, MP grade acrylic shows two strong absorption peaks centered near 298 nm
(region 2) and 339 nm (region 2
′
) due to the absorber package, while both formulations are transparent to wavelengths longer than 380 to 780 nm (region 3), but IAD are calculated at
440 nm for region 3.
4.1
The IAD values for the two exposures and PMMA grades of acrylic are summarized in Table
2
for regions 1, 2, 2
′
, and 3 at equivalent dose steps. UVT acrylic exposures in the QUV and
Newport solar simulator show the UV degradation of unprotected PMMA acrylic (Fig.
4
).
Because this material does not contain a significant UV absorber package, UVA-340 dose is highly effective at degrading this polymer. Photodarkening is shown as a positive IAD curve and is observed at all wavelengths but increases near the fundamental absorption edge.
MP acrylic exposures in QUV and Newport solar simulator show the UV degradation of UV formulated PMMA acrylic (Fig.
5
). This material, which contains a significant UV absorber package, is highly protected from UV degradation. Photodarkening is again observed at the fundamental absorption edge, shown as positive IAD values; however, both studies show some initial photobleaching of UV stabilizers and slight photodarkening in the visible region.
Table 2 Average IAD values for each region in the two stressors and two grades of PMMA acrylic for equivalent dose steps. Equivalent dose steps were 251 MJ ∕ m
QUV accelerated weathering tester and 206 MJ ∕ m
2
2 of UVA-340 dose for the of UVA-340 dose for the Newport solar simulator. IAD values were calculated with Eq. (
7 ), which were at step 4 and step 1, respectively.
Exposures
QUV
Newport solar simulator
PMMA grade
UVT
MP
UVT
MP
Region 1
(275 nm)
0.019
0.0042
0.0069
0.0036
Region 2
(298 nm)
0.0069
− 0
.
00042
0.0021
0.00045
Region 2
′
(339 nm)
0.0029
− 0
.
0018
0.00062
− 0
.
00033
Region 3
(440 nm)
0.00070
7
.
05 × E − 5
0.00011
1.8 e-5
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Murray, Bruckman, and French: Photodegradation in a stress and response framework . . .
Fig. 4 (a) Average induced absorbance to dose (IAD) of ultraviolet transparent (UVT) grade poly(methyl methacrylate) (PMMA) after exposure in the QUV accelerated weathering tester to
UVA-340 irradiance. (b) Average IAD of UVT acrylic after exposure in the Newport solar simulator to AM 1.5 irradiance.
Fig. 5 (a) Average induced absorbance to dose (IAD) of multipurpose (MP) grade poly(methyl methacrylate) (PMMA) after exposure in the QUV accelerated weathering tester to UVA-340 irradiance. (b) Average IAD of MP grade PMMA after exposure in the Newport solar simulator to
AM 1.5 irradiance.
4.2
Yellowness indices are reported for samples exposed to UVA-340 radiation (Fig.
6 ). The yellow-
ing rates for UVT and MP are 0.041 and 0.0035
1
∕ ð
MJ
∕ m
2 Þ
) of UVA-340, respectively. Yellowing rates for Newport solar simulator became non-linear after two dose steps, but yellowing rates in the linear region for UVT and MP are 0.0068 and 0.0014
1
∕ ð
MJ
∕ m 2 Þ
, respectively.
4.3
The temperature of the sample in the Newport solar simulator was approximately 50°C, while the sample temperature in the QUV accelerated weathering tester was approximately 67°C.
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Murray, Bruckman, and French: Photodegradation in a stress and response framework . . .
Fig. 6 Yellowness index comparison of ultraviolet transmitting (UVT) and multipurpose (MP) grade poly(methyl methacrylate) after QUV accelerated weathering tester exposure to UVA-340 irradiance.
5 Discussion
5.1
In our quantification of exposure stresses, UVA-340 dose was chosen as an irradiance dose metric instead of the traditional TUV dose.
29
,
51 , 52
UVA-340 dose is a useful metric because
UVA-340 lamps have peak intensity at 340 nm and their spectral irradiance drops off quickly at wavelengths longer than 360 nm. UVA-340 dose includes the most damaging wavelengths of light that cause degradation in the PMMA samples. The wavelengths between 360 to 400 nm that are included in the TUV metric have not been shown to cause significant degradation, and the spectral match to the light source is not close. Using UVA-340 dose for calculating response rates is expected to show better correlation results of QUV accelerated weathering tester and Newport solar simulator exposures because TUV dose would inherently underestimate the amount of damaging short-wave UV light present in QUV exposure with UVA-340 bulbs (Table
1 ).
5.2
While photodegradation mechanisms are many, complex, and can be ill-defined, photodarkening, photobleaching, and yellowing rates can be discussed without the complexities of explicit mechanistic insights. Higher amounts of total degradation were observed in all samples exposed in the Newport solar simulator, shown by the larger increases in optical absorbance for all samples. This was expected because of the fact that these samples saw much higher net stresses.
However, when comparing responses of samples on a degradation per dose basis, shown with
IAD values, QUV exposure showed a higher rate of degradation (Table
3 ). The QUV accelerated
Table 3 The response ratios of UVT acrylic to MP acrylic for each type of stress. The response ratios are determined from the IAD values in Table
2
for equivalent dose steps between the two stressors.
Stressor
QUV
NSS
Region 1 (275)
4.5
1.9
Region 3 (440 nm)
9.9
6.1
YI
11.7
5.1
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Murray, Bruckman, and French: Photodegradation in a stress and response framework . . .
Fig. 7 (a) Average induced absorbance to dose (IAD) of ultraviolet transmitting (UVT) grade poly
(methyl methacrylate) (PMMA) after exposure in the QUV accelerated weathering tester to UVA-
340 irradiance and after exposure in the Newport solar simulator to AM 1.5 irradiance for equivalent dose steps. (b) Average IAD of multipurpose (MP) grade PMMA after exposure in the QUV accelerated weathering tester to UVA-340 irradiance and after exposure in the Newport solar simulator to AM 1.5 irradiance for equivalent dose steps.
weathering tester is a 5x UV irradiance stress at 1.55 W ∕ m 2 ∕ nm at 340 nm of UVA-340, while the Newport solar simulator is a 50x full spectrum irradiance stress.
The IAD curves for dose steps, which are comparable in terms of net dose between the two exposures (Fig.
7
), show that exposures in the QUV protocol have a larger IAD value in each region than an equivalent dose from the Newport solar simulator. This may be due to the effect of the elevated chamber temperature as summarized in Table
1 . In the UVT acrylic, three times
more photodarkening occurred near the fundamental absorption edge, and six times more photodarkening occurred from 350 to 500 nm. In the MP acrylic, equivalent photodarkening occurred near the fundamental absorption edge, and three times more photodarkening occurred from 375 to 450 nm when comparing the two stressors. However, the stabilizer package shows a much higher level of photobleaching in the QUV exposure protocol, which supports the assumption that the QUV protocol induces more degradation and that this effect may be due to the higher sample temperature. The sample temperatures in the QUV were approximately 67°C and in the
Newport solar simulator were approximately 50°C.
5.3
IAD values for each of the two PMMA grades for each exposure show that the IAD values were higher for UVT acrylic compared to MP acrylic in both stress conditions as seen in Table
2 .
MP shows features in all three regions, while UVT does not have distinct spectral features in region 2. In all samples exposed, there is significant photodarkening occurring in region 1. In
QUV exposure of MP samples there is some initial photobleaching in regions 2 and 2
′
; however, subsequent dose steps show photodarkening of the base polymer lowering this photobleaching, and eventually leading to photodarkening in region 2. Region 3 is characterized by broad band increase in optical absorbance. This response is decreased by a factor of nearly 10 with the addition of the stabilizer package in MP grade acrylic.
In addition to the effects of temperature, another factor which may account for the increased rate of photodegradation in the UVT sample in the QUV is that the sample did not screen any UV light, which passed through the sample. Because samples in the QUV were mounted on aluminum trays, degrading UV light may have reflected, passing through the sample a second time, while in MP samples the UV light was absorbed by the UV stabilizer package, and only lower energy light was reflected. This may account for the differences in IAD rates at the fundamental absorption edge.
The response ratios between the grades show that more degradation occurred in UVT in region 1 and 3 for both stress exposures. This relates to the fact that the UVT acrylic is less stabilized than the MP acrylic. The YI rate was also much higher in UVT than MP in the QUV
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Murray, Bruckman, and French: Photodegradation in a stress and response framework . . .
exposure. Region 2 and 2
′ could not be compared because there are not significant peaks in UVT related to the stabilizer package.
5.4
Changes in yellowness were observed in both acrylic formulations and appear to behave in a near-linear fashion (Fig.
6 ). Changes in yellowness of these materials show the power of YI as a
response because it is very sensitive to small changes in the optical performance of these materials after exposure to these stresses. Yellowing was observed in UVT and MP acrylic for these two types of exposures. Hazing of the samples was not observed. Miller et al. suggested that yellowness and haze are two degradation mechanisms that occur in different classes of PMMA acrylic.
53
The yellowness that occurred in the samples in the QUV exposure occurred throughout the entire sample after the first dose of exposure. The samples in the Newport solar simulator showed the yellowness occurring predominantly on the first surface closest to the irradiation after the first dose. The yellowing proceeded from the first surface to the interior of the 3 mm sample after each subsequent dose. This result agrees with the findings that yellowness occurs at the top surface
54
and can be reduced with polishing.
29
The increased depth penetration of yellowing products in samples exposed in the QUV may be due to the fact that samples were backed with standard aluminum sample holders, which may have contributed to degradation of the samples. This allowed some light to be reflected from the aluminum holders through the samples, though the extent and wavelength dependence of this contribution has not been thoroughly investigated.
Another possibility for the increase in yellowing in the QUV UVA-340 irradiance samples is the higher temperature of the stress conditions. It has been suggested that there is a relationship between UV radiation and temperature when discussing yellowing and that an increase in temperature can increase the yellowing rate.
16
The samples in the Newport solar simulator were placed on a fused ultra-high purity silica photomask blank;
55
therefore, there was very little reflection of light back through the sample, and the temperature of these exposures was less than that of the QUV exposures. The equivalence of UV exposure of the two different sources would also be affected by filter and burner aging in the xenon arc lamp.
56
5.5
While neither of these materials is guaranteed to have a 25-year lifetime in use at 1 sun irradiance for PV applications, the range of their response to stressors can inform material selection criteria for PV applications. UVT acrylic, lacking UV absorbers, can provide insights about the degradation of the base PMMA resin, while MP acrylic shows the effect that a moderate stabilization package can provide. The yellowing rates of these two materials show this sharp contrast, and
MP acrylic shows a 10-fold reduction in yellowing rate. Similar observations can be made about response near the fundamental edge, where MP acrylic shows an approximate 10-fold reduction in photodarkening near the fundamental absorption edge.
5.6
When comparing the ratio of the IADs between the two exposures of equivalent doses, the QUV exposure has a much higher acceleration rate of degradation compared to the Newport solar simulator exposure (Table
4 ). The acceleration rate in the significant regions for each grade
of PMAA is different, which can be caused by test conditions, composition of material additives, degradation mechanism as well as temperature. The observation that a constant
“ acceleration
” factor was not observed in the different degradation regions (1, 2, 3) and mechanisms demonstrates that the traditional search for a single acceleration factor applicable to a particular material exposed under differing stresses or stress levels is probably an unreasonable expectation. This supports the need for developing a more versatile R
ð
S
Þ framework in which to encompass stressors, stress levels and multiple responses.
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Murray, Bruckman, and French: Photodegradation in a stress and response framework . . .
Table 4 Ratios of average IAD values from QUV (step 4) to Newport solar simulator (step 1) for each region for the two grades of PMMA of equivalent dose steps.
UVT
MP
Region 1 (275 nm)
2.8
1.2
Region 2 (298 nm)
3.3
1.6
Region 2
′
(339 nm)
4.6
0.86
Region 3 (440 nm)
6.3
3.8
5.7
Use of a R
ð
S
Þ framework is an important way to study, understand, and cross-correlate lifetime and degradation studies of materials, components, or systems. By studying degradation responses due to multiple types of exposure conditions and steps, we are able to understand linear, sub- and supra-linear, and nonlinear responses over a wide range of stress levels and types of stressors along with the combination of stresses and time-varying stresses. Also, by comparing or cross-correlating the effects of multiple stressors to the response of a single stress exposure, we can understand the specific (combined or sequential) synergistic mechanisms at play in a material system. Significant understanding of the ways a material may behave over its service life can be understood by comparing results of real-world exposures to the stress and response framework of the system that has been developed beforehand. In cases where unwanted degradation is observed, the stress and response framework can be used to understand and guide mitigation of these unwanted responses. Additionally, because a stress and response framework can include highly accelerated stress conditions, without assuming that these conditions map linearly to in-service exposures, useful information about end-of-life performance can be gleaned.
50 , 57
,
58
6 Conclusion
Two grades of acrylic PMMA with different concentration of UV stabilizers were exposed to stresses in order to understand the stress-response framework for PMMA acrylic. By using IAD and YI as response metrics and comparing these responses across material systems in the same class, and across different stress conditions, valuable insights into the lifetime performance of these materials can be understood. Both MP and UVT acrylic show similarities in their degradation with exposure to simulated sunlight and UVA-340 fluorescent weathering. The materials respond to the fluorescent exposure at a higher rate than in the simulated solar exposure; however, this variation may be due to differences in sample temperature during exposure. The QUV exposure showed to be a more aggressive weathering test than exposure to the Newport solar simulator, being three to six times more damaging to UVT acrylic and one to three times more damaging to MP acrylic on a per-dose basis. QUV showed more yellowing, while a ten-fold reduction of these phenomena was observed with the addition of the stabilizer package in
MP acrylic. Exposure doses achieved were equivalent to one to eight years of dose when compared to the UV-only and full-spectrum dose of AM 1.5D in Arizona, while these tests were performed in less than 40 days. Further study into degradation mechanisms of acrylic will elucidate the exact phenomena that contribute to the material responses to stress. This framework and these insights are being applied to other acrylic grades for Fresnel lenses and mirrors.
Continuing work will require identification and quantization of stressors, identification of ways to meaningfully accelerate stresses and reproduce naturally occurring degradation mechanisms, and application of this framework to other stressors such as temperature, humidity, mechanical load corrosive environments, and applied loads. An understanding of the stress-response framework in the PV L&DS will allow for the same framework to be applied to real-world environmental conditions to enable indoor testing to be correlated to outdoor lifetimes.
Further work will entail the monitoring of other responses, such as haze, bidirectional scattering distribution function, and contact angle measurements of these two PMMA grades, as well
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Murray, Bruckman, and French: Photodegradation in a stress and response framework . . .
as other grades of PMMA, after stress exposure. In addition, dark recovery time and humidity will be investigated in a multifactor stress experiment.
Acknowledgments
The authors acknowledge funding from Ohio Third Frontier, Photovoltaic Program Award Tech
11-060. Research was performed at the SDLE Center at Case Western Reserve University, funded through the Ohio Third Frontier, Wright Project Program Award Tech 12-004. We appreciate the diligent efforts of Esther Deena at the SDLE center, Mark Schuetz of Replex
Plastics, Sean Fowler of Q-Labs. Sample tracking and data management support was provided by REDCap
TM
,
59
funded by the Clinical and Translational Science Collaborative (CTSC) grant support (1 UL1 RR024989 from NCRR/NIH).
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Murray, Bruckman, and French: Photodegradation in a stress and response framework . . .
Myles P. Murray is a graduate student at Case Western Reserve University in the Department of Materials Science and Engineering. He received a
Bachelor of Arts in chemistry from Hendrix College
’ s School for Green
Environmental Chemistry. Before attending CWRU, he was an environmental monitoring specialist with the Division of Air Quality in Cleveland,
Ohio. His research is focused on improving the cost of energy from of solar energy through enhanced optical materials for photovoltaic systems and improving bankability of PV systems through lifetime and degradation science.
Laura S. Bruckman is a research associate at CWRU. She received her
PhD in analytical chemistry from the University of South Carolina in Dr.
Michael L. Myrick
’ s research group in August 2011. Her research while at
University of South Carolina was on the rapid classification of phytoplankton cells using single-cell fluorescence measurements and multivariate optical computing. Her work since joining Dr. Roger French
’ s group at
CWRU has been on the degradation mechanisms of acrylic and acrylic back surface mirrors in order to better understand the Lifetime and
Degradation Science of these materials.
Roger H. French (F. Alex Nason Professor, Department of Materials
Science and Engineering) joined CWRU in August 2010 after 24 years of conducting basic research and product development in DuPont
’ s Central
Research. He is the director of the Solar Durability and Lifetime Extension
Center at CWRU. He has a broad experience in developing and commercializing optical materials for many different applications and in optimizing these materials for improved radiation durability and lifetime. He is also a nationally recognized expert in Lifetime and Degradation Science (L&DS) for commercial applications, evidenced by his work on attenuating phase shift photomasks, fluoropolymer pellicles for photolithography, immersion lithography imaging fluids, and materials for concentrating photovoltaic systems. He has 22 issued patents and more than 145 publications.
Journal of Photonics for Energy 022004-17
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Vol. 2, 2012