Service Life Prediction of Organic Coatings - American Chemical Society

Chapter 18

Interfacial Composition of Metallized Polymer Materials after Accelerated Weathering

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 13, 2017 | http://pubs.acs.org Publication Date: April 15, 1999 | doi: 10.1021/bk-1999-0722.ch018

D. E . King and G. J. Jorgensen National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, C O 80401

Correlations between changes in the macroscopic performance and in the microscopic properties experienced by weathered materials can be used to identify degradation mechanisms and suggest new formulations that exhibit improved durability. Of particular interest is the relationship between optical reflectance and interfacial composition of metallized polymer solar mirrors. A sample test matrix of silvered polymethyl methacrylate (PMMA) mirrors was prepared to systematically study changes in interfacial composition with accelerated exposure and how the chemical changes influence the reflectance of the mirrors. The samples were subjected to accelerated exposure testing in a XENOTEST 1200 LM environmental chamber. Spectral hemispherical reflectance was measured as a function of exposure time, and selected samples were removed at various exposuretimesto allow surface analysis of the silver/PMMA interface.

There is a large potential market for lightweight, highly reflective mirrors for use in solar energy concentrating systems. Metallized polymer constructions can provide low-cost, highly reliable reflectors. To be cost effective for commercial power production, the solar-weighted specular reflectance of the mirrors must exceed 90%. The reflective layer of choice for this application is silver (/). Ideal polymer reflectors must remain stable for more than 20 years in an outdoor service environment, be easily cleaned, and require little maintenance. Commercially available silvered polymethyl methacrylate (PMMA) films (in which, for example, PMMA is used as the superstrate) combine excellent outdoor weatherability with high transparency and UV stability (2). Understanding the correlation between accelerated weathering and realtimein-service environmental exposure will allow accurate prediction of service lifetime for these materials. The determination of failure mechanisms as a function of accelerated weathering

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Bauer and Martin; Service Life Prediction of Organic Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


289 is a starting point in understanding real time field failures and can help direct future reflector research efforts. Research to improve the performance of this class of reflector materials has been a continuing priority within the solar thermal electric industry and at NREL (3). This study was initiated to elucidate the mechanism of loss in reflectance at the silver/polymer interface of two sample constructions. An extensive review of possible reactions and failure mechanisms at the silver/polymer interface is provided in (4). A variety of mirror materials currently being tested include two different PMMA constructions: construction I (PMMA/Ag/Adhesive), and construction II (PMMA/Ag/Cu/Adhesive), shown in Figures 1 and 2, respectively.

Construction I P M M A 89 |Jm

Adhesive 13 ym

F i g u r e 1. M i r r o r construction I is produced by evaporating 0.1 m m o f silver onto P M M A sheet. T h e metallized film is then coated w i t h an acrylic adhesive. B y r e m o v i n g a release liner the film can be laminated to any surface producing a highly specular mirror.

The transparent PMMA typically has up to 2 wt% UV absorbers that effectively screen out most high energy photons. However, because silver is transparent to UV light around 320 nm, the addition of a metallic copper layer behind the silver further reduces the UV flux incident on the unstabilized acrylic adhesive, thus providing further protection against possible light-induced reactions in the adhesive layer that could have deleterious effects on the reflective layer. These different constructions can fail during real-world deployment in two different ways: the first type of failure is corrosion of the metallic silver, which typically results in a steady loss of reflectance as a function of exposuretime;and the second failure is a catastrophic delamination that occurs at the silver/polymer interface upon exposure to excessive moisture. Construction II is much more resistant to delamination failure than construction I. During accelerated weathering, both constructions fail because of loss in reflectance. It is possible that for the copper backed mirror (construction II), copper metal diffuses to the polymer interface, resulting in improved metal/polymer adhesion at the reflective interface.


290

Construction II


P M M A 89 Mm

Figure 2. M i r r o r construction II is similar to construction I except that 0.05 mm o f copper is evaporated onto the silver before the acrylic adhesive is applied to the metallized f i l m .

This behavior is important as it significantly reduces delamination failure of the material in the field. Construction II also has an extended effective lifetime relative to construction I, as shown in Figure 3, where the solar weighted hemispherical reflectance remains above 90% for nine weeks longer in the accelerated exposure test than construction I. The two constructions have been found to exhibit different corrosion failure modes. The copper backed material tends to be highly delamination resistant and displays increased lifetime, but after a time the mirror does fail. To produce an improved reflector, it is important to understand why construction II exhibits an increased lifetime and what the ultimate failure mechanisms are for both constructions. A "corrosion" failure mode of the copper backed material may involve the diffusion of copper metal through the silver reflective layer and subsequent buildup at the silver/polymer interface as a function of accelerated exposure. This copper buildup may then contribute to the loss in reflectance of the mirror. In addition, the copper metal may then react with the oxygen functionality of the PMMA, or perhaps simply oxidize in the presence of water and oxygen that diffuse through the PMMA. The copper oxides formed would then further contribute to the drop in performance of the mirror. An increase in the sulfur concentration was found, as a function of weathering, at the silver/PMMA interface in construction I (NREL internal reports, 1989-1997). It is well known that silver metal does not oxidize in air, but sulfur compounds will react with and corrode silver surfaces (4-6). The origin of the sulfur and the precise nature of the chemical interaction at the reflective interface is not well understood. It is suspected that sulfur species migrate to the silver/PMMA interface from the PMMA, the bulk adhesive, or both. The metallic copper layer in construction II may serve as a diffusion barrier or getter and slow the transport of corrosive sulfur species to the reflective silver interface. This


291 may explain the increased lifetime of construction II mirrors. To correlate the observed optical degradation and interfacial composition with accelerated weathering, X-ray photoelectron spectroscopy (XPS) was used to determine the chemical composition, including copper and sulfur concentrations, at the reflective interface as a function of exposure time.


Experimental Two sample sets of 50 samples (44.5 mm x 66.7 mm coupons) each consisting of the two different reflector constructions were prepared, optically characterized, and introduced into an accelerated weathering chamber. The XENO chamber uses a filtered xenon arc light source to closely reproduce a terrestrial air-mass 1.5 solar spectrum at an intensity of about two suns. Samples were exposed at 60°C and 80% relative humidity (RH). Some samples were positioned in the weathering chamber with half of the reflector surface shielded from the light source to provide control samples exposed to the same temperature and RH, but without any incident light. The sample set consists of construction I and construction II both laminated to bare aluminum substrates, as shown in Figures 1 and 2, respectively. For both constructions, the thickness of the acrylic adhesive layer is 13 |im and the silver reflective layer is 0.1 p,m, and for construction II the copper layer is 0.05 |im thick. Both constructions were produced by vacuum evaporating the metals onto biaxially stretched 89 |im thick PMMA film. All samples were removed from test and the reflectance was measured every two weeks. A random subset of samples was optically characterized during the intervening week. Reflectance was measured with a Perkin-Elmer UV/VIS/NIR Lambda 9 spectrophotometer. In addition, selected samples were also periodically removed from test to allow XPS analysis of the reflective (silver/polymer) interface. This interface was accessed by cutting small (10 mm x 14 mm) coupons from the reflector samples and soaking them in deionized (Dl) water for a few minutes to several hours. The water intrudes between the evaporated silver layer and the polymer substrate resulting in a significant reduction in the silver-polymer adhesion. After the water soak, the clear PMMA film can be removed intact, exposing the reflective surface. Once this interface is exposed the samples were immediately placed into the vacuum chamber for analysis to prevent atmospheric contamination of the silver metal surface. Although the water soak introduces some uncertainty as to the identity of the actual interface being analyzed, it is the only practical way of exposing the silver/polymer interfacial region in these mirrors. XPS data were acquired with an LHS-10 surface analysis system using a 250-W non-monochromatic Mg Ka X-ray source, previously described (7). The base pressure of the vacuum chamber typically was in the 10" Torr range, but during the analysis of these polymer samples the system pressure increased to about 3X10" Torr from the outgassing of the polymers. The peak heights from the survey XPS spectra were used to determine the elemental composition of the 10

8


292 surface of the reflector and thus to follow changes in the interfacial composition of the mirrors as a function of accelerated exposure testing.


Results and Discussion The two reflector constructions perform equally well until about the seventh week of accelerated testing when construction I begins to degrade at a faster rate than construction II. This trend continues until about week 20 when the two constructions again degrade at equivalent rates. Construction I falls below the critical reflectance value of 90% by week 14, while construction II remains above 90% for 22 weeks. The copper backed construction clearly has an increased stability during accelerated exposure, as shown in Figure 3.

Reflectance as a Function of XENO Exposure -•—Construction II

1

a ^

f

-•—Construction 1

8

l i 9

O CO

0

1

| _ _—

1

1

1

1

1

1

4

1

1

1

2

4

6

8

10

12

14

16

18

20

22

24

26

Time of XENO Exposure Time (weeks) Figure 3. The two mirror constructions perform equally well until about the seventh week of accelerated weathering, when I begins to degrade at a greater rate than II. After about 20 weeks, the rates of degradation again become roughly equivalent. Construction II has a reflectance above 90% for about 9 weeks longer than construction I.

XPS survey spectra of the silver interface of both constructions are shown in Figures 4 and 5 after 20 weeks of accelerated weathering. In general, there is an increase in the surface carbon content and a concomitant decrease in silver. The increase in carbon appears to result from the loss of the sharp silver/PMMA (reflector) interface. Decomposition of the PMMA or migration of decomposed adhesive components through pinholes in the silver layer may be responsible for the increase in carbon at the reflector interface. The parts of the samples that were shielded from the light showed very little change in reflectance or surface composition as a function of time in the accelerated test.


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XPS Survey Spectrum of the Reflector Surface of Construction I After 20 Weeks Exposure

1000

800

600

400

200

0

Binding Energy, eV Figure 4. After a brief soak in deionized water, the transparent P M M A is easily removed from the reflective layer allowing X P S analysis of the silver surface. Atomic concentrations from the survey spectra indicate that after 20 weeks of weathering there is a 20% decrease in surface silver, a 5% loss in oxygen, a 5% increase in sulfur, and a 20% increase in carbon.

XPS Survey Spectrum of the Reflector Surface of Construction II After 20 Weeks Exposure 01s

2 O Auger

^-^vL

Cu Auger | C1s

Ag3d I Vl

Cu 2p

1

1000

800

...i.

600

S2p

i

1

400

200

0

Binding Energy, eV Figure 5. The reflective interface of weathered construction II has a different surface atomic composition than construction I. After 20 weeks, the surface carbon has increased by only 6%, sulfur has decreased 3%, and silver has decreased 35%, whereas the surface copper remains constant. The minimal loss of reflectance (5%) after 20 weeks and the low surface silver content may be the result of degradation of the silver/polymer interface.

There is a direct correlation between reflectance and the surface concentration of silver in construction I for the first 11 weeks of accelerated weathering. After this time other mechanisms of degradation appear to dominate the loss in reflectance. This is evident from Figure 6, where the surface silver


294 concentration tends to stabilize after 11 weeks of weathering but the reflectance continues to decrease.

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Service Life Prediction of Organic Coatings - American Chemical Society

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