Prepared by the Jct Propulsion Laboratory, Ca.lifornia Institu te of Technology, for the Department of Energy by agreement with the National Aeronautics and Space Administration .

The J PL Low-Cost Solar Array Project is sponsored by the Department of Energy (DOE) and forms part o f the Solar Photovoltaic Conversion Program to initiate a major effort toward the development of low-cost solar arrays.

This report was prepared as an account of work sponsored by the United States Governmen t. Neither the United States nor the Uni ted States De partment of Energy, nor any of their employees, nor any of their contractors , subcontractors, or their employees, makl-s any warranty, express or implied, o r assumes any legal liability or responsibi lity for the accuracy, complctcncs.5 o r usefulness o f any in fo rma tio n . apparatus, product or process disclosed , or reprcscnrs that its use would no t infringe privately owned rights .

Low-Cost Solar Array Project

5101-77

Photodeg radation of Polymeric Encapsulants of Solar Cell Modules

A. Gupta

August 10, 1978

Prepared for

Department of Energy

by Jet Propulsion Laboratory California Institute of Technology Pasadena. California

ABSTRACT

This report deals with the mechanisms of photodegradation of encapsulant materials in solar cell modules. Data have been presented on irradiation sources, their applications in simulative or accelerated testing or mechanistic studies, and their calibration. In discussing mechanisms, the emphasis has been on the possible application of these mechanisms in creating models which correlate a change in molecular structure to changes in physical properties, which, in turn, control performance in the field. For example, photooxidation of silicones has been shown to yield hydroxyl groups pendant on the siloxane chain which increases the polar character of the silicone surface, as indicated by surface energy analysis. A change in the surface polarity of silicones directly affects their interfacial bond strength to module substrates and causes weakening of bond strength to hydrophobic surfaces. Experi­ments prove that such a UV weakened bond may undergo delamination on exposure to moisture.

A major section of the report is devoted to acrylic photochemistry and the role of UV stabilizers and screening agents, because they are viewed as potential low-cost encapsulants capable of outdoor perform­ance for 20 years or more. In addition to reviewing some of the rich literature in these areas, we have described some inhouse work of pre­liminary nature. It is hoped that the addition of these recent results will add a topical flavor to the report. Among the acrylics we have studied are the methacrylates, the acrylates, and their copolymers which often possess unique photochemical properties not ascribable to either of the two copolymers by themselves.

The focus of the discussion has always been pottants, since in our view the pottant is the central element in encapsulation design, and material choices for cover and substrates must take into account the protection the pottant needs. A low-cost pottant, for example, may need protection from UV, necessitating the addition of UV screening agents in the cover. Polyvinyl butyral needs a hermetically sealed en­vironment in order to function outdoors without degradation. Hence, it must be sealed between two pieces of glass or other material impervious to moisture and oxygen. The choice of adhesives and primers is also based partially on the choice of the pottant. An example is provided by our work on RTV-615. It is found that RTV-615 (70 mil) will delami­nate with ultraviolet light followed by water soak if either RTV 108 or QC 36-060 is used as primers. Thus, adoption of the material science approach to life prediction dictates that, initially, we study the mech­anism of photodegradation of potential candidates for pottants.

ii

CONTENTS

I. INTRODUCTION -------------------------------------------- 1-1

II. SPECTRAL RADIANCE OF LAMPS AND THEIR CALIBRATION --------- 2-1

III.

IV.

V.

A. LIGHT SOURCES-------------------------------------- 2-1

B. FILTERS-------------------------------------------- 2-4

C. CALIBRATION OF LIGHT SOURCES----------------------- 2-6

PHOTODEGRADATION OF SILICONES AND POLYVINYLBUTYRAL -------

A. PHOTODEGRADATION OF SILICONES----------------------

B. PHOTODEGRADATION OF POLYVINYLBUTYROL --------------­

PHOTODEGRADATION OF ACRYLICS-----------------------------

3-1

3-1

3-4

4-1

A. INTRODUCTION--------------------------------------- 4-1

B. POLYMETHACRYLATES ---------------------------------- 4-1

C. POLYALKYLACRYLATES --------------------------------- 4-10

D. COPOLYMERS OF METHACRYLATES AND ACRYLATES ---------- 4-12

ULTRAVIOLET STABILIZERS---------------------------------­

A. INTRODUCTION---------------------------------------

B.

1.

2.

3.

C.

ULTRAVIOLET ABSORBERS AND QUENCHERS----------------

Benzophenones as Ultraviolet Stabilizers----------­

Nitrogen-containing Stabilizers

Other Photostabilizers -----------------------------

ANTIOXIDANTS---------------------------------------

5-1

5-1

5-1

5-3

5-5

VI. CONCLUSION ----------------------------------------------

5-8

5-9

6-1.

6-3 REFERENCES-----------------------------------------------------

iii

Figures

2-1. U-H-I Strong Electric Company - 175 amp------------ 2-2

2-2. Peerless Magnarc - Strong Electric Company -60 amp -------------------------------------------- 2-3

2-3. G.E. High Pressure Xenon - 1740 W ----------------- 2-4

2-4. Solar Irradiance at Air Mass One------------------- 2-5

2-5. Hanovia Hg-Xe - 1500 W ----------------------------- 2-6

2-6. Photochemical Reactor I---------------------------- 2-7

2-7. Photochemical Reactor II--------------------------- 2-8

2-8. Pyrex Filters: Thickness vs Cutoff Wavelength----- 2-9

2-9. Transmission of Schott Glass Filters--------------- 2-10

2-10. NBS Filter-phototube UV System -------------------- 2-11

2-11. Average Incoming Radiation on Mount Wilson for 30-minute Intervals (by R. Stair and J. S. Nader)--------------------------------------- 2-12

2-12. Wavelength Response of JPL Actinometer------------- 2-15

2-13. Correction Factor (vida supra) for Broadband Radiation ------------------------------- 2-16

2-14. Correlation of Rate of Photodegradation of PMMA in Two Different Photoreactors Using the UV Integral Measured by JPL Actinometer-------- 2-17

3-1. RTV-615 Photodegradation Under AM-1 and Shorter Wavelengths-------------------------------- 3-2

3-2. Loss of Si-H Stretch in RTV-615 ------------------- 3-5

3-3. Absorption Spectrum of RTV-615 ---------------------3-4. Absorption Spectrum of PVB UV-40

(Monsanto) in Methanol -----------------------------4-1. Absorption Spectra of Acrylics ---------------------4-2. Electronic Absorption Spectrum of Photoproduct

in PMMA Medium-pressure HG Arc 5 Min

3-6

3-7

4-3

Irradiation --------------------------------------- 4-6

iv

4-3. Rate of Formation of Photo product in Pt'l}lA -------------------------------------------- 4-7

4-4. Volatilization of Polymethyl Hethacrylatc Exposed to Different Sources of Ultraviolet Radiation------------------------------ 4-8

4-5. Decrease of Molecular Weight of Polymethyl Methacrylate as a }'unction of Irradiation Period -------------------------------- 4-9

4-6. Volatilization on Irradiation of Polyalkyl Methacrylates ---------------------------- 4-U

4-7. Chain Scission in Polyalkyl Methacrylates as a Function of Irradiation Period---------------- 4-12

4-8. Volatilization Rates of Polyacrylates 4-13

4-9. Activation Spectra of Acrylics--------------------- 4-14

4-10. Molecular Weight Distribution as a Function of Irradiation Period for P-(!!_buA) and P-(_!_buA) ------------------------------------------- 4-15

4-11. Extent of Volatilization in 30 Min at Various Temperatures---------------------------- 4-16

4-12. Volatilization-time Curves for the Photodegradation of Polymethyl Methacrylate) and Methyl Methacrylate-Methyl Acrylate Copolymers at 170°C --------------- 4-17

4-13. Degree of Degradation vs Exposure at 30°C ---------- 4-18

4-14. Dependence of Rates of Scission and Crosslinking on the Composition of ~~1A/MA Copolymers Irradiated in the Form of Films--------- 4-19

5-1. First Order Decay of Photoenol Transient Absorption on Excitation of a Solution of 2-Hydroxygenzophenone in Benzene with a Nitrogen Laser (337 nm)---------------------------- 5-6

5-2.

5-3.

Absorption Spectra of some Benzotriazoles

Synergism between a Disulfide and a Phenolic Antioxidant in Polyethylene

V

5-7

5-12

Tables

2-1. Radiation from Medium-pressure Mercury Lamp-------- 2-7

3-1. Film Surface Energy Characteristics --------------- 3-3

4-1. Quantum Yields of Photodegradation 4-3

vi

SECTION I

INTRODUCTION

Deterioration of performance of encapsulants during outdoor exposure can be related to damage caused by a temperature rise or by temperature cycling, absorption of solar ultraviolet radiation, reac­tion with oxygen, absorption of moisture, (which may or may not be fol­lowed by hydrolysis), reaction with pollutants, interaction of the cover with adhering dust particles, or two or more of these weathering elements acting synergistically. This report deals with the photode­gradation of common encapsulant materials, which may be caused by solar UV acting along (photolysis), solar UV and oxygen acting together (pho­tooxidation), or solar UV and moisture acting together (photohydroly­sis). Damage to encapsulants leading to loss of performance in the field is usually a complex combination of degradative changes, and, consequently, it is difficult to separate the effects of various types of hostile environments from one another on a whole encapsulation pack­age. Therefore, it is easier to analyze the total damage in terms of elementary (or incremental) changes taking place in the materials con­stituting the encapsulation package; changes caused by photolysis, photo­oxidation, or photohydrolysis. The rates of these processes may be measured in the laboratory where the particular environment may be duplicated or modified in a predetermfned manner. A mechanism of pho­todegradation of each material is obtained through these rate meas­urements, as well as measurements of rates of other processes which do not occur outdoors, but knowledge of which is needed for construction of a comprehensive model of photodegradation of that material. Outdoor measurements of these rates should be made whenever possible, primarily to calculate acceleration factors being achieved at the laboratory. The mechanism (or model) represents the course of chemical change in each material taking place under both natural and artificial weathering environments, and it can be related to changes in mechanical and elec­trical properties, and properties peculiar to interfaces or surfaces.

These links have to be developed for each material and tested through concurrent experiments, carried out in the laboratory and out­doors, in which well behaved and well understood chemical changes, such as generation of functional groups, evolution of gases or change in molecular weight distribution are plotted against physical/mechanical properties such as modulus and glass transition temperature, which in turn are linked to performance characteristics such as dirt retention, peel strength and transmission of visible light. These relationships would then allow us to construct a model of degradation of a certain encapsulation package, measured in terms of cell power output if the outdoor environmental variables (temperature, UV, etc.) are continuously monitored. 1 Building such comprehensive models is thus seen to be an in­volved process and should be attempted to test the validity of the pro­posed methodology and to predict lifetime of significant designs of encapsulant packages. The submodels for each encapsulant material should however be constructed without waiting for specific designs to develop, since they can subsequently be fitted to any proposed module design.

1-1

The candidate encapsulants dealt with in this report are silicones (RTV-615 and Silgard), polyvinylbutyral, and acrylics. The discussion includes state-of-the-art knowledge on mechanism of photodegradation of each of these materials (only significant mechanistic pathways and deg­radation data have been included, without making any attempt at being comprehensive) and results obtained at JPL. Data are scarce on PVB and silicones; a substantial amount of literature exists on photodegrada­tion of acrylics. We have also included brief sections on radiation sources and their calibration, and mechanism of operation of ultraviolet stabilizers. Light sources and optical filters have to be chosen care­fully, since photodegradation processes are often wavelength dependent. Calibration of these sources has to be performed frequently since few sources are stable, and both intensities and wavelengths change as the lamps accumulate hours of usage. Choice of light sources is discussed in terms of experimental design, i.e., whether simulation of AM-1 sunlight is desired, whether a hyperacceleration of solar ultraviolet (295 - 360 nm) is needed, or whether a short wavelength ultraviolet (250 - 300 nm) exposure is planned. Ultraviolet stabilizers have been used to stabilize polymers such as polypropylene or polystyrene for outdoor use and may be used to stabilize low-cost transparent polymers for module encapsulation functioning either as a cover or as a pottant. In the disscussion on UV stabilizers, we have focussed on quenchers (of excited states) and UV screens because the best stabilizers should trap and deactivate electronic excitation energy in the polymer before this energy has had an opportunity to cleave a bond. Thus, radical scav­engers and antioxidants may often be useful in certain systems, but in general they can only retard the rate of photodegradation and get consumed as they function. Sometimes the products then photosensitize and accelerate photodegradation of the polymer after the stabilizer is used up.

1-2

SECTION II

SPECTRAL RADIANCE OF LAMPS AND THEIR CALIBRATION

A. LIGHT SOURCES

Solar simulators or weather-ometers have been widely used to monitor degradation caused by solar ultraviolet, either by itself, or acting in conjunction with oxygen, humidity (including humidity cycling) and temperature cycling. The objective is to predict lifetime of mater­ials or systems such as polymeri.c films or coatings on specific sub­strates. The older weather-ometers frequently used carbon arc sources, a typical energy output vs wavelength of two of which are given in Figures 2-1 and 2-2.2 Carbon arc sources are quite unstable;2,3 this is caused by the nature of the arc spectrum, which consists of many line spectra super-imposed upon a continuum. Certain types of carbon arc sources are also quite deficient in ultraviolet (< 340 nm) radiation, relative to solar irradiance in this region. They also tend to be richer in infrared than high pressure Xenon arc lamps which aggrevates sample cooling problems, when accelerated testing is desired.

High pressure Xenon arc lamps are much more stable and suitable for use as irradiation sources in weather-ometers and solar simulators. Figure 2-3 shows energy output vs wavelength plots for such a source. These measurements were reported by Duncan et al2 who measured the light output using a Beckman DK-1 spectrophotometer calibrated against stand­ard lamp traceable to NBS. These light sources will have to be suitably filtered in order to reproduce approximately the shape of solar irradi­ance at air mass one, shown in Figure 2-4.4 A good (better than 10%) fit requires frequent careful calibrations of the lamp and measurement of spectral transmission of filters and screens to be used. Commer­cially available solar simulators may be used for routine work, partic­ularly if they can be calibrated at reasonable intervals according to NBS specifications.

The construction and design of solar simulators ultimately depend on the use they will be put into. Thus, for photodegradation studies, wavelength vs intensity distribution in the ultraviolet and visible part of the spectrum is the most important parameter which has to be simu­lated as precisely as possible. For other applications, total energy per unit area is more important.

Two other common light sources are the high pressure mercury Xenon arc and the medium pressure mercury arc. The high pressure mercury Xenon arc lamp generates a continuum on which are superimposed lines due to mercury and Xenon (Figure 2-5). This ultraviolet rich source may be filtered to eliminate wavelengths shorter than 294 nm and used for high acceleration of the solar (AM-1) ultraviolet in the region 294 - 340 nm. However, a considerable amount of infrared emission is generated by this lamp; hence cooling problems arise when such an accelerated test is car­ried out. The medium-pressure mercury lamp is considered to be the best ultraviolet source for the simulation and acceleration of solar ultra­violet radiation. These lamps are quite stable (<10% drop in output

2-1

600 1mr \ \ . -

~I \\ r\ I

::t l I .. ~ 100 :\j ·-~

N ...... _."' I e

60 u "' 0 ~[ ~ LU

u z <(

I 0 <( I

°' 101

_, <(

°' I-u w Q. 6f VI

3

j 1 l

f 500 1000

. "'·--·, .............

1500

WAVELENGTH (Nanometers)

. ......_ ·-· ............. ............. __ -·.._·--·-·

2000 2500

Figure 2-1. U-H-I Strong Electric Company - 175 amp

after 1 year of use),5 relatively inexpensive, and easy to install. The low intensity in the infrared region may be further attenuated by filtering through solution filters. Here again, short wavelength ultraviolet must be removed if outdoor photodegradation is to be simu­lated or accelerated. Table 2-1 gives output vs wavelength for this type of light source.6

Suitable photochemical reactors may have to be designed for pre­cise rate measurements or mechanistic studies. A rotating photochem­ical reactor was constructed by Moses et al.,7 which uses medium pres­sure mercury lamps and may be used to measure rates of photodegradation at 313 ±10 nm and 366 ±10 nm. A modified system has been designed and constructed by us which uses a high pressure Xenon lamp (1.6 - 5 kW) and can measure rates in up to thirty samples simulataneously, each enclosed

2-2

600

300

-I r\. I• ..... ..... :t. ·~ ....

.... .. .......... . ..._ ~

N

100

60

/-~ ,- . . .......... . . --·--I

8 :?

1 w u z < c ;$

30

10

6

3

• -•

! ·"·~ 1 •-,...,,,,. . ""-. ."

soo 1000 1SOO 2000 WAVELENGTH (Nanometers)

·, . .__ ..........

'

2500

Figure 2-2. Peerless Magnarc - Strong Electric Company - 60 amp

in a different environment (parameters: light including wavelength and intensity, temperature, ambient gas composition). This system is con­tinuously calibrated with a silicon radiometer, and can be also used for broadband irradiation of large (6 x 6 in.) samples. Details will not be presented here, but are available on request. A smaller .rotating photo­chemical reactor has also been designed (Figure 2-6) in which the light source is a 1 kW high pressure Xenon lamp which may be focussed by moving the arc along the axis of a paraboloidal mirror. The focussed beam is ·incident on the outer surf ace of a rotating aluminum cylinder which functions as a sample holder. Here all the samples experience the same environment during one run, and it is difficult to carry out experiments under an inert atmosphere. Figures 2-6 and 2-7 show the layouts of photochemical reactors in our laboratory.

2-3

10,000

6000

3000

-I :t.

' 1000 ... G) t;

N 600 I

E u

i .!. 300 UJ u z < 2i ~ ...,

100 < a:: ... u w 60 0.. V)

A _):,n. I~ ~~ I .

I

i ''\; 1. \;\ A

·,iv\ ........... ~

30 ....__

'· ............. ---·--. 500 1000 2000 2500

WAVELENGTH (Nanometers)

Figure 2-3. G.E. High Pressure Xenon - 1740 W

B. FILTERS

Appropriate filters will have to be used with any of the above­mentioned sources to remove radiation below 293 - 295 nm. A good choice is Pyrex whose cut off wavelength vs thickness is shown in Figure 2-8. An optical quality Pyrex filter loses 5% or more light on each face due to reflection and scattering, so that the best transmis­sion observed is about 90%. For shorter wavelength cut offs, Corex or Vycor filters may be used. Infrared emission may be removed by using either a filter cell filled with distilled water or a dilute solution of NiS04.6H20 (50/1).8 The nickel sulfate filter will transmit up to 85% of light at 335 nm, but will cut off (<5%) by 365 nm. Hence, solar

2-4

0.20

N 0.15 I ~ -I E ~

-< :I: 0.10

WAVELENGTH - nm

(From Reference 10)

Figure 2-4. Solar Irradiance at Air Mass One

ultraviolet gets through NiS04 solutions, but for purposes of strict simulation distilled water must be used. Isolation of the 295 - 330 nm region may be accomplished by using a solution of KzCr04 (0.5 g/lt) and NazC03 (0.1% by wt) in distilled water.9 Figure 2-9 shows spectral transmission of a series of Schott glass filters.10 Filters should be maintained at as close to room temperature as possible since the spec­tral transmission may be a function of temperature. Interference fil­ters are of limited use in this area for this reason. Glass filters should be checked for deterioration of quality due to solarization.

Although solution filters listed in standard references and glass filters are suitable for most measurements, sometimes it is necessary to prepare special solution filters for cut offs in particular spectral regions. For example, a solution of the polymer itself may be used as a filter to isolate the effects of solid state interactions on the pho­tochemical processes, and study the photochemistry taking place in tails

2-5

-I :t. • .. .!

N"' I E u

:E D

.!. w u z ,c(

0 ~ ~ 0:: t­u w CL Cl'I

10,000

6000

3000

1000

600

300

100

60

30

• I

• • •

• • •

r •.

•

•

500 1000 1500 2000 2500

WAVE LE NGTH(Nanometers)

Figure 2-5. Hanovia Hg-Xe - 1500 W

of band envelopes arising out of these interactions. Thus, proper selection of optical filters may be an important aid in studying photo­chemical processes in polymeric encapsulants.

C. CALIBRATION OF LIGHT SOURCES

The two commonly-used methods of calibration of a light source are (1) radiometry and (2) actinometry.

2-6

Table 2-1. Radiation from Medium-pressure Mercury Lamp

Wavelength Energy Output Wavelength Energy Output nm watts nm watts

222.4 3.7 270.0 1.0

232 .0 1.5 280. 4 2.4

236.0 2.3 289 .4 1.62

238.0 2.3 296.7 4.3

240.0 1.9 302. 5 7.2

248.2 2.3 313.0 13.2

253.7 5.8 334 .1 2.4

257.1 1. 5 366.0 25.6

265.2 4.0 404. 5 11.0

Spectral distribution of 450 watts medium pressure mercury lamp manufactured by

Hanovia, total output 222.4 - 1367.3 nm is 202. 7 watts.

HIGH PRESSURE XENON LAMP

.,...._ ROTARY REACTOR ~---of BAND OF SAMPLE HOLDERS

MOVABLE MONOCHROMATOR ELLIPSOIDAL MIRROR

Figure 2-6. Photochemical Reactor I

2-7

HEATER

OPTICAL FILTERS ~ GAS OUTLET (GLASS, LIQUID, ETC)

SAMPLE HOLDER

ROTATING HIGH PRESSURE XENON

·LAMP

DRIVE MECHANISM (360° OSCILLATION)

,? 11 , , ' ( I

',( 11

l~

PHOTOCHEMICAL REACTOR

Figure 2-7. Photochemical Reactor II

SAMPLE HOLDERS 3 LEVELS 8 PER LEVEL

Radiometry. Radiometric calibration essentially involves measurement of photocurrent in a light sensitive material when light is incident on it. Since quantum yield of photocurrent in these materials can never be known at all times with sufficient precision, radiometric calibration is always relative; in other words a radiometer is usually used to calibrate a source by comparing its irradiance in a certain bandwidth with that from a reference source, usually traceable to NBS. The optical system of a spectroradiometer may include a quartz hemisphere enclosing the slits, a quartz diffuser or an integrating sphere coated with BaS04, or MgO, and filters mounted on a filter wheel or a mono­chromator with high thruput. The drive mechanism of the filter wheel or the monochromator may be automated so that a voltage vs wavelength plot is obtained. Figure 2-10 shows a design developed and constructed at NBS for measurements of solar ultraviolet.11,12 In each case frequent calibFation of the phototube with a standard lamp is essential.

2-8

3000

• • • 900k Attenuation

2900 000 990k Attenuation 0 0 .0

2800 0 o,c( • :c 5 • z 2700 • w 0 .... • ~ ~

2600

2500 • 0 1.0 2.0 3.0 4.0 5.0

THICKNESS mm

Figure 2-8. Pyrex Filters: Thickness vs Cutoff Wavelength

Arveson13 has carried out detailed radiometric measurements of solar ultraviolet aboard an aircraft, and his measurements may be compared with data published by Johnsonl4 and Nicolet.15. JPL has acquired a spectroradiometer utilizing a monochromator for measurements of ultra­violet radiation from light sources. It is currently undergoing cali­bration and testing. Figure 2-11 shows a plot of intensity vs wave­length obtained by Stair and Naderll for solar ultraviolet.

Actinometry. Another common method of calibration of light sources is known as actinometry. In this technique, the incident light is absorbed by a well understood photochemical system which undergoes a photochemical reaction whose rate can be monitored easily and precisely. The quantum efficiency of the process is usually available in literature, or may be measured using a calibrated or standard source. For best results, the quantum efficiency should be independent

2-9

80

60

20

10

0.10

CURVE SYMBOL

a

b

C

d

e

f

9

h

i

i k

I

220 260 300 340 380 420 460 500 540 580 620 660 700 7 40

WAVLENGTH - nm

THICKNESS FOR CURVE THICKNESS FOR GLASS TYPE CURVE (mm) SYMBOL GLASS TYPE CURVE (mm)

WG 280 1 m GG 475 3

WG 295 1 n GG 495 3

WG 305 1 0 OG 515 3

WG 320 1 p OG 530 3

WG 335 1 q OG 550 3

WG 345 1 r OG 570 3

WG 360 1 s OG 590 3

WG 395 1 t RG 610 3

GG 400 3 u RG 630 3

GG 420 3 V RG 645 3

GG 435 3 w RG 665 3

GG 455 3 X RG 695 3

Figure 2-9. Transmission of Schott Glass Filters

2-10

QUARTZ HEMISPHERE

~ 9863

PICO AMMETER

INTEGRATING SPHERE

VOLTAGE GENEVA DIVIDER MOTOR AND GEAR DRIVE RECORDER

110 AC

Figure 2-10. NBS Filter-phototube UV System

of wavelength. The following general scheme may be applied to calculate the absorbed light intensity:

hv * A > A

* k1 A > A

* k2 A > B (II.2)

Scheme 2-1

Then

dB k2 - = .,.----,.- X I = "' I d t k2 + k1

'I'S • ·

When ~Bis the quantum yield of formation of B, and I is the intensity of light being incident on the system. If dB/dt is measured in moles/ liter second, I is obtained in einsteins/second which when multiplied by N, the Avogadro's number gives photons/second. Hence, strict band

2-11

N

~ ~ z Q ~ a ~ C> z ~ 0 u z

7.0 • ... ...

6.0 • • • ...

• ... ... ...

s.o

• • • Oct6

• • • Oct 16 4.0

• ... 3.0

2.0

1.0 ...__ ______ ...__ __ __,__ __ ___..___ __ _,__ __ --J ___ ....,,__ __ --L, __ ____J

aro ~ ~ ~ ~ ~ ~ 380 390

nm

Figure 2-11. Average Incoming Radiation on Mount Wilson for 30-minute Intervals (by R. Stair and J. S. Nader)

400

isolation (usually within± 5 nm) is needed to obtain accurate values of energy deposited in watts. Characteristics of a good actinometric system are

(a) The photoproduct B should be thermally stable and should have a low extinction coefficient relative to A.

(b) The quantum efficiency should be wavelength- and temperature-independent, and be high.

(c) It should be easy to quantitate B.

Actinometers are best suited to measure light intensities con­fined within a narrow wavelength band, since the concentration of A may then be adjusted so that A absorbs more than 99% of the incident radia­tion. If a broadband source is to be measured so that A does not absorb all light incident on the system, a correction factor has to be

2-12

applied which determines the accuracy of the measurement. This correc­tion may be defined as

Then

a= I/Io when I is the light intensity absorbed, while Io is the light intensity incident on the system.

Ai a= t AA. 6Ai, when AAi is the absorption (in fraction or per­

centage) of the 1 light absorbed at wavelength Ai• This is simply the area under the transmission curve after correcting for scattering losses. Examples of actinometric systems are the uranyl oxalate,16 ferrioxalate,17 and benzophenonepentadiene.18

The uranyl oxalate actinometer involves the following photo-· reaction:

and is suitable for wavelengths below 350 nm. The ferrioxalate actino­metry involves the photoreaction:

hv > 2 ~e++ 2CO r' + 2 --------- (II.5)

and can be used for wavelengths shorter than 475 nm. The quantum yields stay constant at 1.23 .± 0.02 up to 366 nm and then drops to 1.03 at 436 nm. The benzophenone- Cisl,3-pentadiene actinometry uses the scheme below:

cp2co . hv > ( <P2 co) 1 * (II.6)

(<P2 co) 1 * > ( <P2 co) 3* (II. 7)

3* ( <t>2 CO) + ci s - D ~ c1>2CO + D 3* (II . 8)

o3*~ a. ci s - D + ( 1 - a.) trans - D (II .9)

Scheme 2-2

2-13

Here Dis 1,3-pentadiene, ~2co is benzophenone, and the superscript denotes the spin multiplicity of the excited state. a is the decay ratio of the diene triplet. The quantum yield is $trans-D =ax 1.0, since the ketone singlet intersystem crosses to the ketone triplet with unit efficiency. This relation holds only for very low (<3%) conver­sions, since as trans pentadiene builds up in the system, it starts intercepting ketone triplets and going back to the cis isomer. For larger conversions the equation

R.n 55.5 (II. IO) 55. 5 - Fobs

should be used, when F0

is the corrected and Fobs the observed conver­sion to trans pentadiene. This system may be used for wavelengths shorter than 340 nm.

Recently JPL has developed a film actinometer function according to eq. II. 11.

@__.N02

I

............. CHO

hv > ~--NO

~"-..... COOH

(II.11)

Pitts et al, 19 studied the above system in fluid solution and employed it to measure solar ultraviolet. It was demonstrated that the quantum efficiency of the system is 0.50 even when it is incorporated in a polymer film and that it has a very low (negligible) energy of activa­tion, i.e., the thermal coefficient of the photoreactivity, is too low to be measured in the range of 200 - sooc. The response of the acti­nometer as a function of wavelength is given in Figure 2-12. Fig-ure 2-13 gives the correction factor (vide supra) for broadband radia­tion monitored by the JPL actinometer. Theoretically this correction factor holds for either sources which emit the same intensity over all wavelengths, or if the same source is being measured by using actino­meters of two different thicknesses. The overall light absorption efficiency in the range 290 - 450 nm may be calculated for each thick­ness of polymer film and loading of the photosensitive material. Using a series of films one may obtain wavelength resolution of the incident radiation. This system is particularly useful for measurement of solar ultraviolet integrated over time and over specific surfaces, including otherwise inaccessible interfaces. It generates a UV integral which may be used to correlate photodegradation rates better than units cur­rently used such as "Sun hours" or "UV hours".

2-14

90

80

70

60

z 0 vi Vl

~ Vl 50 z ~ I-

~ 0

40

30

20

lO

o~;..._ ______________ J...-______________ ___.

300 400 500 nm_..

Figure 2-12. Wavelength Response of JPL Actinometer

2-15

0.580

0.560

0.540

0.520 E C

0

~ 0.500 I 0 II') ('I

> 0.480 => C w a:i

°' 0.460 0 ti) a:a

N <( I .... .._. z 0.440 °' w

~ w Q..

0.420

0.400

0.380

0.360 0.400 0.500 0.600

0.150 0.200

0.700 0.800 Ir ABSORBANCE

0.250 2

mg NBA/m

0.300

0.900 1.00

0.350 0.400

Figure 2-13. Correction Factor (vida supra) for Broadband Radiation

1.10 1.20

0.450

Various types of usage may be envisioned for the actinometer. For routine measurements of solar ultraviolet in the range 290 - 450 nm in remote sites over specified time periods (1 min - 1 month) and over specific surfaces, it would be convenient to use a set of matched acti­nometer films (same thickness, same composition). For wavelength reso­lution, it would be necessary to use a range in thickness or concentra­tion or both. Figure 2-14 shows a typical photochemical reaction being followed actinometrically using two different types of light sources.

8

4

• • • XENON ARC LAMP

~ MERCURY ARC LAMP BOTH UNFILTERED

1.84 3.68 7.32

mg OF NBA CONVERTED/ cm

18.40 2

Figure 2-14. Correlation of Rate of Photodegradation of PMMA in Two Different Photoreactors Using the UV Integral Measured by JPL Actinometer

2-17

SECTION III

PHOTODEGRADATION OF SILICONES AND POLYVINYLBUTYRAL

A. PHOTODEGRADATION OF SILICONES

Little is known about photodegradation rates and mechanisms of siloxanes. RTV-615 which is polydimethylsiloxane marketed by General Electric, and Sylgard 184 a siloxane of similar structure (Dow Corning) have been investigated by Springborn labs. We have recently initiated a study of photo chemistry of RTV-615 in thin films. Springborn results20 indicate that both materials are susceptible to photohydroly­sis. Springboro reported that transmittance of both RTV-615 and Sylgard 184 decrease dramatically if they are subjected to ultraviolet radiation in presence of 100% moisture. This finding implies that photohydrolysis generates unsaturation in the polymer, a mechanism for which is yet to be proposed. We carried out photolysis of RTV-615 under an inert gas atmosphere as well as air (humidity O - 25%) and we applied ultraviolet radiation in the spectral region 250 - 300 nm as well as 300 - 350 nm. Figure 3-1 is a plot of increase of absorbance at 3400 cm-1 attributed to generation of aliphatic hydroxyl groups as a function of radiation period, using the high pressure Xenon arc and the medium pressure mercury arc radiation sources. It (and other data) indicates that in case of this material exposure the mercury arc consti­tutes a valid technique for acceleration. It is estimated that in the region..::_ 330 nm, the mercury arc can provide acceleration factors of up to 1000 suns (AM-1). The data in Figure III.! represents an acceleration factor of 320 (over AM-1 sunlight) for the mercury arc and about 8 for the Xenon arc. It is to be noted that these acceleration factors are applicable for this material only, since these are affected by the absorption spectrum of the material. Using the same configuration, we calculate an intrinsic acceleration factor of 650 ±65 for the mercury arc and 16 ±4 for the Xenon arc in the wavelength range 290 - 380 nm. The formation of aliphatic hydroxyl groups may be attributed to photo­oxidative cleavage of Si-C bonds. The same type of growth of absorption due to aliphatic hydroxyl groups is observed when pyrex filtered high pressure Xenon arcs are used as a source of radiation.

CH200H

I OH

I -0 - Si - 0 - -0-Si -0- ~ -0-Si -0-

1 1 CH 3

CH3

Scheme 3-1

3-1

o.sr-------,--r---------.,...---------,,---------~--------,~------....,...---------....... --------

I E u

0

~ ~ w u z ~ ~

0.4

0.3

0 ~ 0.2 <(

o. l

0

MERCURY LAMP (MEDIUM PRESSURE)

4 8 12 16 20 24

TIME OF IRRADIATION, hr

Figure 3-1. RTV-615 Photodegradation Under AM-1 and Shorter Wavelengths

3-2

28

Photooxidation may take place either through attack by oxygen (ground state) on the excited state of siloxane, or more probably by the attack of singlet oxygen on ground state siloxanes. The singlet oxygen may be generated through impurity sensitization. In any case, it may be noted that little color change is expected and practically none is observed (a very small amount of gain is noted in some samples). The appearance of aliphatic hydroxyl groups particularly on the surface of the film is expected to change its surface energy and bonding charac­teristics. Rockwell Science Center has made some preliminary surface energy measurements on samples prepared at JPL, and the results are given in Table 3-1.20

In Table 3-1, a and Bare measures of surface polarity and dispersiveness respectively.

Thus

These measurements clearly show that the polar character of the surface is increasing, while the nonpolar character is decreasing. These results imply that bonding of RTV-615 with nonpolar (hydrophobic) surfaces should decrease as photooxidation proceeds. This prediction will be checked by doing peel strength measurements, and if verified will form the mechanistic basis of observations that certain modules containing silicones as pottants undergo delamination on outdoor exposure.

Sample No.

1

2

3

4

Table 3-1. Film Surface Energy Characteristics

Exposure Period (hrs} a k (X320 acc. factor (dynes/cm) 2

over AM-1 sunlight)

0 4.82

0 4.80

6 4.68

24 4.57

3-3

a k {dynes/cm) 2

1.21

1.11

1.44

1.51

On longer term irradiation with short wavelength ultraviolet we detected cleavage of Si-H bonds, the rate of which is shown in Fig-ure 3-2, and then a gradual growth of carbonyl groups characterized by increased absorption at 1710 cm-1. On continued irradiation the carbonyl groups decrease in concentration and the aliphatic hydroxyl groups become the dominant absorbing species in the polymer. Electronic absorption spectra (for a typical spectrum see Figure 3-3) are essen­tially unaffected which is not surprising, since we were studying 1-mil films and aliphatic nonconjugated carbonyl groups have extinction coefficients less than 100 in the 270 - 310 nm region. Studies are continuing with a view to determine the nature of the impurity sensi­tizer (which could be the residual curing agent in the film) which triggers the photooxidation reaction, and also the effect of environment on the rate of photooxidation and the mechanism of development of unsaturation in the polymer.

B. PHOTODEGRADATION OF POLYVINYLBUTYRAL

Reports on photodegradation of polyvinylbutyral are rare in published literature. It appears that PVB undergoes thermal oxidation at significant rates at or above its glass transition temperatures which ranges from -20 to 55°C. The rate, and perhaps the mechanism, of oxidation is altered by the addition of plasticizers, stabilizers, and other additives. Commercial polyvinylbutyral contains a consider­able amount of polyvinyl formal which is even more susceptible to oxidation. There are reports that the oxidation is catalyzed by light, i.e., photooxidation may also take place over and above thermal oxida­tion. Photodegradation data available from Springborn20 Laboratories indicate that both oxidation and photooxidation is severe at 55°C and 90% RH. Samples aged at 55°C (air oven) retained clarity, although phy­sical properties (e.g., modulus, elongation and tensile strength) deter­iorated. The intrinsic material is reportedly not prone to hydrolysis, but the impurities in the polymer are. Chemical structure indicates that it should be vulnerable to acid hydrolysis. All of these degrada­tive processes can be eliminated if the polymer is used in a hermetically sealed environment, i.e., between two pieces of glass with the edge sealed off with a suitable sealant. Used in this manner, it has found very large-scale application as a laminate in safety glasses for automobiles and airplanes. A spectrum of commercial PVB (Saflex made by Monsanto) dissolved in methanol is given in Figure 3-4. Its photochemistry is being studied both in film and in methanol solution under an inert atmosphere and under air (or oxygen) at JPL.

3-4

>-I-vi z w 0 ...J < u i= 0.. 0

1.0

1 mil RTV-615

0.8

0.6

0.4

0.2

0--------'------------..1.....-------------' 2300 3100

WAVELENGTH, angstrom

Figure 3-2. Absorption Spectrum of RTV-615

3-5

3900

0.5

I

5 0.4 0 ~ N .... <(

w u z i g ICO <(

0.3

0.2

O.J Si-H

MEDIUM PRESSURE MERCURY LAMP, NO FILTER

QL-----------'-----------'---------..._, ________ __. 0 2 3 4

IRRADIATION PERIOD, hr

Figure 3-3. Loss of Si-H Stretch in RTV-615

3-.6

1.8

VI I-

z :::)

>- 1.5 a::: <(

I= co a::: <(

~ u z <( l.2 CCI a::: 0 VI CCI <(

360 380 400 nm-.

Figure 3-4. Absorption Spectrum of PVB UV-40 (Monsanto)

3-7

A. INTRODUCTION

SECTION IV

PHOTODEGRADATION OF ACRYLICS

Acrylic photodegradation mainly involves chain scission, crosslinking, evolution of volatiles and development of conjugation. Photooxidation may be important in certain systems, while there is little evidence in literature of enhanced hydrolysis due to exposure to solar ultraviolet. A potentially important, but little explored, area is secondary photoprocesses sensitized by absorption of light by volatiles which are initially evolved by the polymer on scission of side chains or pendant groups.

B. POLYMETHACRYLATES

The polymethacrylates have the general structure

-cH -2

CH 3 I C -

I COOR

CH -2 When R =

CH 3 methyl

CH2cH 3 ethyl

- ~H2)2cH 3 propyl

The presence of the pendant methyl group has several important effects on photo chemistry. Firstly, absence of an abstractable hydrogen eliminates photooxidation process as :i'9 eqn IV.1

H OOH

I hv :;Iii

I -cH -C CH - -CH c- CH -

2 ' 2 02 2

I 2

COOR COOR

! 0 II

CH2 - C - CH- + -CH - C + H 0 IV.I I 2 2

I 2

COOR COOR

4-1

Secondly, the methyl group increases the glass transition temperature and lowers polymeric chain mobility. This affects the rate of radical chain processes, makes radical recombination more probable, and slows down certain radical disproportionation routes. Literature data are most abundant on PMMA (polymethylmethacrylate), which has been subjected to several careful outdoor and indoor weather-ometer tests and to mechanistic investigations. The absorption spectrum of PMMA is given in Figure 4-1. This spectrum indicates that pure PMMA does not absorb any solar ultraviolet (AM-1), and hence is expected to be photostable outdoors. Its excellent thermal and hydrolytic stability is attested to by the fact that PMMA has been exposed outdoors for 17 years without significant degradation.22 Isakson23 studied several acrylics including PMMA in different kinds of weather-ometers as well as outdoors in Miami, Florida and Lamont, Illinois. His results con­firm the reported outdoor stability of Pl~1A. Specifically, PMMA did not undergo any significant chemical change on 2000 hours continuous exposure in a weather-ometer, and 9 months exposure in both outdoor sites. He also observed that PMMA is resistant to degradation by ozone, which attacked phthalate and isophthalate polyesters and styren­ated acrylic copolymers. However, PMMA is not an inherently photostable polymer, although it may be a weatherable polymer at ground level. Table 4-1 gives quantum yields of photodegradation of various polymers (in solid state) on irradiation with 2537 A light which is absorbed by PMMA.24 As is apparent, PMMA appears to be a photo-reactive polymer under these conditions, undergoing degradation faster than polystyrene and polyethylene. These results imply that one has to be careful in designing simulative experiments to test acrylic encapsulants. Unless the source of radiation is suitably filtered to resemble AM-1 sunlight, the results might be completely in variance with what one would observe outdoors.

The primary effect of UV radiation on PMMA is to cause chain scission; the radicals

ultimately appearing in the system. The overall characteristics of the photochemical process depends upon the subsequent reaction of these radicals.ZS

4-2

100

80

z g 60 Ill,. ~

0 V, al < 40 ?fl.

20

10

Polymer

Polymethyl Acrylate

PMMA

PVC

Po 1 ys tyrene

Polyacrylo-nitrile

e PMMA (3 mil) b. PButadiene grafted on

MMA - St - Acrylamide Copolymer (1 mil)

CORRECTED FOR FRONT SURFACE REFLECTION AND SCATTERING LOSSES

250 260 270 280 290 300 310 320 330

nm

Figure 4-1. Absorption Spectra of Acrylics

Table 4-1. Quantum Yields of Photodegradation

Wavelength Quantum Yield of Quantum Yield of 0

A Chain Scission Gas Evolution 39

· 2537 1. 3 X 10-2

30 2537 {2.5 ) -2 2.3 X 10- 4 at 313 nm3

3 + 1. 5 X 10

24 2537 1.1 X 10-2

2537 9 X 10-S 4.3 X 1 -2 0, 3.5 X 10-4

2537 {2-7. 7) X 10-4

4-3

At high temperature (relative to Tg) the polymer is in liquid state, monomer produced in the equilibrium (IV.2) can easily escape, so that the reaction shown below (eq. IV.2) tends to move to

CH 3 CH 3

CH 3 CH 3 I I I I

CH - C - CH C· ____;:,.. CH2 - C· + CH2 C --IV.2

2 I 2 I ~ I I

COOCH 3 COOCH 3

COOCH 3 COOCH 3

the right and quantitative conversion to monomer tends to occur. Hence, depolymerization is a photo-initiated process which is rate-limited by diffusion of monomer molecules through the polymer at low tempera­tures. When the polymer is a rigid solid, the depolymerization rate is very slow and the photolysis is characterized by chain scission.26,27 The following mechanism may be used to interpret the outstanding obser­vations of primary photoprocesses reported in literature on PMMA pho­tolysis as a function of temperature.

* (IV.3) -®- ~ -®

CH 3

* I (IV .4) -®- ---+ ·C - CH -

I 2 COOCH 3

-P·

M + - Q· {IV.5)

p • ------+

p. ____. • COOCH3

+ CH 3 {IV.6)

I CH CH -

CH 3

* I (IV.7) -®- -CH -C-CH - + ·OCH 3

--------=> 2 I 2 . co

CH 3 * I

_(IV. 8) -©- .,. -CH -C- CH2 - + · COOCH 3 2 •

Scheme 4-2

4-4

Here eq. IV.3 represents some type of excitation process, while IV.4 represents transfer of this excitation energy to the carbon skeleton so that chain scission can take place. There is some controversy as to the nature of the chromophore in PMMA, which may be end groups or the ester sidechain. Maccullum28 has given some experimental evidence that the ratio of quantum yield of chain scission and quantum yield of monomer formation shows concentration dependance. This result indicates that an energy transfer process is delivering electronic energy to the backbone. Eq IV.S represents depolymerization and monomer formation, while eq. IV.6 shows how macroradicals may be stabilized through dis­proportionation. Eqs. IV.7 and IV.8 represent photo reactions of the ester functionality and rationalize observed formation of CO, CO2, methanol, hydrogen, and formaldehyde on photolysis of PMMA.29,30 Recently we observed some spectral changes in PMMA films on short-term exposure to short wavelength ultraviolet radiation which can be inter­preted as follows:

CH 3 I

CH - C - CH -2 2 I

·CO

)

CH2 C CH 3 I

a C CH2

+

CH 3 I CH - C·

2 I COOCH 3

The development of this a, S unsaturated carbonyl group is shown in Figure 4-2. This is an unusual result and must be considered prelim­inary until we verify it by using PMMA which has been purified by GPC (preparato.ry). * The rate of formation of this unsaturation is shown in Figure 4-3. Weight loss data have been obtained under helium and oxygen and are linear with radiation period, as expected for low con­versions. Figure 4-4 shows these plots obtained with the high pressure Xenon arc lamp in the rotating photochemical reactor and with the medium pressure mercury arc (both unfiltered). Both plots are linear, which indicates that the mercury arc may be used as an ultraviolet accelerator. The acceleration obtained in this case is 45.0 for the mercury lamp relative to the Xenon lamp. The samples were then

*Subsequent work indicated that the a, S unsaturated carbonyl group was arising out of an impurity, probably some thermal oxidation product in PMUA films. The primary photo product is now identified to be a carbonyl chromophore on the basis of UV-visible and FT-IR data on purified films irradiated in a nitrogen atmosphere.

4-5

w u z ~ 0::: 0 V,

~

0.6

0 '::""250 _________ 300'----------3-50L.,_ ________ 400__J

nm

Figure 4-2. Electronic Absorption Spectrum of Photoproduct in PMMA Medium-pressure Hg Arc 5 min Irradiation

analyzed by GPC, results being shown in Figure 4-5. It is apparent that PMMA does not undergo crosslinking on photodegradation, which agrees with data in literature and which is in contrast to what is observed in other poly alkyl methacrylates.

Three types of quantum efficiencies may be defined: the quantum efficiency of primary bond cleavage which should be temperature inde­pendent or nearly so, quantum efficiency of chain scission which takes into account geminate recombination of radical pairs, and quantum yields of various product formation. By comparing the quantum yield of monomer formation with that of chain scission one may obtain kinetic chain length of radical propagated photodepolymerization. Cowley and Melville31 estimated the primary quantum yield to be 0.1 while Charlesby et a1,32 and Fox37 reported that at room temperature, in rigid polymer, the quantum yield of chain scission is about 1.2 x 10-2 in vacuum and 4 x 10-2 in air or nitrogen. This means that geminate recombination is about 60% efficient at room temperature, a plausible

4-6

E C

1.0

0.9

0.8

~ N 0.] f-<C w u z ~ 0::: 0 0.6 V') iCIC <C

0.5

0.4

ABSORBANCE MONITORED AT 250 nm

0.3~_,. ____________ ...... ____________ ...... ____________ ...._ ____________ ..._ _________ ..._ ____________ ..._ ___ ...,

0 10 20 30 40 50 60 TIME OF tRRADIAT ION, min

Figure 4-3. Rate of Formation of Photoproduct in PMMA

estimate. Kinetic chain length is also strongly dependent on chain mobility or temperature. It is estimated to be between 2 x 103 and 2 x 104 at 170°C, while Fox37 shows that it is about 5 at 20°C. In fact, at room temperature ester side chain photolysis and chain scission predominate over depolymerization, since quantum yield of methanol formation is 0.48 at 20°C on irradiation with 2537 A ultraviolet.37 At high temperatures both photo and thermal degradation yield carbon dioxide in 1:1 ratio with chain scission, while room temperature photolysis yields more CO2 than chain fragment products. This is again attributed to ester photolysis, which may turn out to have a profound influence on the mechanical properties of PMMA films photodegraded at

4-7

12

8

4

WT LOSS OF PMMA

e-e-e XENON LAMP !::::.-!::::.-!::::. MERCURY LAMP BOTH UNFILTERED

80 120 160

ACCUMULATED UV DOSAGE Arbi tra ry Uni ts

200 240

Figure 4-4. Volatilization of Polymethyl Methacrylate Exposed to Different Sources of Ultraviolet Radiation

280

room temperature. Ester photolysis products are expected to diffuse into PMMA and act as plasticizers. Depolymerization (i.e., monomer evolution) does not change the molecular weight appreciably, so Tg and associated mechanical properties are not affected. This might reconcile the observation of Riabov,31 et al, that the quantum yield of gas evolu­tion from PMMA is nonzero at 313 nm, with results of outdoor exposure tests carried out at Scandia21 which indicate no significant change in physical properties after 17 years of exposure. In our tests we find that the reported quantum yield at 313 nm is an overestimate. On the other hand, chain scission reduces molecular wiehgt systematically and lowers the glass transition temperature.

The general excellent weathering characteristics of PMMA make it a good choice for an encapsulant material. Its high glass transition temperature and hardness makes it unsuitable for a pottant, but ideal

4-8

POL YMETHYL METHACRYLATE, PMMA, "PLEXIGLAS"

MOLECULAR WEIGHT DECREASE AS FUNCTION OF

IRRADIATION PERIOD. ....,----450,000

UNFILTERED LIGHT FROM HIGH PRESSURE XENON ARC, RUN IN AIR, 25°C /

HOURS

•-•-•o &-&-&2 6.-~-6. 6, 5

0-0-019

17

4800 10,000

16 15 14 13 12

INCHES

50,000 97,000 171,000 -402,000

POLYSTYRENE CALIBRATION

Figure 4-5. Decrease of Molecular Weight of Polymethyl Methacrylate as a Function of Irradiation Period

11

for use as a cover material. Some cautionary statement may be in order here. It has been reported that PMMA photodegradation may be sensitized by impurities or photo products which may absorb at longer wavelengths than PMMA does, and so pick up energy from AM-1 solar ultraviolet radiation. Therefore, addition of a UV screening material (absorber) to PMMA should be carried out only when such addition does not initiate photodegradation of PMMA. We added 2,4 - dihydroxybenzophenone, a well known UV screen and quencher to PMMA (mol. wt. 461,000) so as to make a blend containing 10 wt% stabilizer. The resultant film was clear and showed no variation from spot to spot. This was irradiated with unfil­tered light from mercury arc which would normally degrade PMMA quite rapidly (Figure 4-4). Weight loss was negligible over a period of 28 hours (<1%), which indicates a stabilization efficiency of more than 0.975 if stabilization efficiency is defined as 1 - ~1/$2 when $1 is the quantum efficiency of degradation when stabilizer is present and $z is the quantum efficiency when there is no stabilizer. However, spectral data indicated some photo reaction of the stabilizer/screen, which manifests itself as increase in absorbance at wavelengths in the region 350 - 400 nm. We are currently investigating the nature of this photoprocess.

4-9

Other polyalkylmethacrylates have been studied, but not as extensively as PMMA. In general rate of volatilization increases with length of side chain, as shown in Figure 4-6. The exception is that P (n-butylmethacrylate) has a higher rate of volatilization than P (lauryl methacrylate), which could be due to the fact that the bulk of lauryl group promotes radical recombination while hydrogen abstraction and consequent disproportionation is easy with a pendant n-butyl group.34 Another important degradation parameter is chai;;- scission per molecule which is calculated from number average molecular weight of the sample. When this is plotted vs period of exposure to ultraviolet, it shows the relative effects of disproportionation of the macroradical it unzipping to monomeric units and recombination leading to crosslinking. When these processes are monitored separately, e.g., by measuring rate of monomer formation, or gel fraction increase, one can construct a reliable model of reactivity of the macroradical, and photoreactivity of the system. Figure 4-7 shows a plot of chain scission per molecule vs time of irradiation for certain poly alkyl methacrylates. As can be expected, this index is quite dependent on temperature. At temperatures low compared to Tg, the reaction is predominantly chain scission, while at high temperatures depolymerization or monomer production becomes the dominant mode of reaction.

C. POLYALKYLACRYLATES

The polyalkylacrylate photochemistry is different from methacrylate photochemistry in several important respects. Crosslink­ing is an important photo process, while chain scission or ester photolysis can result in volatilization. Figure 4-8 shows a plot of volatilization vs irradiation period for various polyacrylates.34 Here again, rate of volatilization increases with the side chain, indicating that side chain (ester) photolysis becomes important. Figure 4-9 shows wavelength dependence of photodegradation of polyacrylates.34 It is interesting that the photoreactivity persists up to 310 nm. This result is of obvious importance when acrylic elastomers are used as pottants, since these elastomers consist of polyacrylates or copolymers containing polyacrylates. Abstractable hydrogens and steric bulk of pendant methyl groups strongly affect the rate of crosslinking. Thus, poly n-butylacrylate forms a stable crosslinked network which undergoes chain scission and crosslinking at equal rates. Poly isobutylacrylate on the other hand, undergoes chain scission more rapidly and so the gel frac­tion drops with time of irradiation. The molecular weight distributions of the two systems are given in Figure 4-10.

Photolysis of polymethylacrylate39 results in evolution of formaldehyde,40 methanol, methyl formate and CO2. CO, methane and hydrogen were also detected but no methyl acrylate,34 which indicates that depolymerization reaction does not take place. Fox36,37 observed that the number of scissions per molecule is independent of intensity of radiation in vacuum which he interpreted to mean that polymer radicals were terminating through combination with small, relatively mobile radicals in the vicinity of the break point rather than through recombination of polymer radicals.

4-10

30

25 0

2537 A RADIATION

e-e-e P (!!-Bu MA)

0-0-0 P (EMA)

6-6-6 PMMA

20

~ .!l .... 3 ~ 0

z 0 15

~ 3 0 >

10

5

0 1,,,C;.. _____ ......_ _____ -'--_____ ......._ _____________ ____

0 50 100 150 200 250

IRRADIATION TIME, hr (From Reference 34)

Figure 4-6. Volatilization on Irradiation of Polyalkyl Methacrylates

4-11

w ...J :, u w ...J

~ ~ z 0 v; V')

0 Vl

1 .0

0.5

0

0

........ P-(2 EHA)

000 P-{!!-bu A)

~ P-EA

... P-M.A

10 20 30 40 50 60 70

IRRADIATION TIME, hr (From Reference 34) Figure 4-7. Chain Scission in Polyalkyl Methacrylates

as a Function of Irradiation Period

D. COPOLYMERS OF METHACRYLATES AND ACRYLATES

Only one copolymer has been studied up to date which is the poly methylmethacrylate-methyl acrylate system. Grassie, Torrance and Colford27 studied this system at 170° and found that random chain scission results in depolymerization but the rate is strongly dependent on copolymer composition (Figure 4-11). The zero slope line obtained with PMMA indicates that the temperature.dependence observed with the copolymers is not a viscosity effect, but is due to temperature dependence of some chemical rate processes in the copolymers. The ratio of monomers evolved is methyl acrylate vs methyl methacrylate in 1:10 ratio. Volatilization-time curves were obtained, and are shown in Figure 4-12. AllisonJ8 reported on some room temperature photolysis of methyl methacrylate-methyl acrylate copolymer, and his results are shown in Figure 4-13. He sought experimental evidence that chain scission in PMMA is due to secondary photolysis of photochemically generated aldehyde groups. Grassie26 measured degradation rates as function of methyl methacrylate concentration in the copolymer (Figure 4-14) and showed that the rate of chain scission in the methyl

4-12

20

...... P-{2EHA}

000 P-{~-bu A)

15 ~ P-EA

::R .... P-MA 0

V1 V1 0 ..J

i .. z 10 0 j::

~ ..J

3 0 >

5

0 50 100 150 200 250

IRRADIATION PERIOD, hr (From Reference 34)

Figure 4-8. Volatilization Rates of Polyacrylates

methacrylate rich polymers is much greater in solution which he attributed to the high Tg and low mobility of PMMA. Volatilization plots (e.g., Figure 4-12) can also be constructed as a function of ~ethyl methacrylate concentration and these curves have been published for two copolymers, P(M?1A-MA) and P(MMA !!_-but A). In both cases the acrylate units strongly inhibit the unzipping process, and volatiliza­tion (monomer evolution) is only appreciable when MMA content is higher than 50%. However, initiation of chain scission is a random process, therefore depropagation is halted only when unzipping reaches an acrylate unit. This is the mechanism which Grassie proposed in order to interpret data on scission, crosslinking and volatilization rates obtained on photolysis of copylymers of methyl methacrylate and methyl acrylate or methyl methacrylate and n-butyl acrylate. This is given below as Scheme IV.3. Butyraldehyde-is also found to be a product in the P(MMA, !!_bu-A) system which may form from a side chain radical.

4-13.

30 • Poly n-butyl Acrylate 0.90 0 Poly 2-ethyl hexyl Acrylate

w · 6 MMA - Styrene - Acrylonitrile u Copolymer + grafted on Polybutadiene z ~ 0.75 o== 0 -V, I c:o e <( C,

z ~ 0 20 0.60 ~ co a:: I-<( <( u >-0 !:: 0::: V, 0 0.45 z >- w :c 0 z _.

<( z u 0 10 0.30 t: I- 0 u :, 0 w 0:::

?fl. 0.15

O---~---~-~-~--~---~---~--~---~o 200 240 280 320 360 400 440 480 520

nm (From Reference 35) Figure 4-9. Activation Spectra of Acrylics

15 20 25

5 10 15

6 0 hrs 0 12 hrs e 20 hrs

6 0 hrs D 20 hrs • 40 hrs 0 80 hrs • 200 hrs

30

20

P-(!J-bu A)

35

P-(l bu A)

25

Figure 4-10. Nolecular Height Distribution as a Function of Irradiation Period for P- (~buA) and P-(i_buA)

(From Reference 34)

4-15

~ 0 .. z 0

< ~ ...J .... <( ...J

0 >

50

------------D--~0>-----0

40

30

20

10

o _ ___, ________ ....._ ___________ ......_ ____________ _. 140 150 160

TEMPERATURE, 0 c 170 180 190

(From Reference 27)

Figure 4-11. Extent of Volatilization in 30 Min .at Various Temperatures; 0-0-0-PMMA; a-m-a-MA-MMA copolymer (1: 26) A-·A··.&-MA-MMA copolymer (1: 7)

4-16

~ 0

z 0 i= ~ ...J

t-

~ 0 >

80

70

60

50

40

30

20

10

0 20 40 60 80 100 120 140 160

TIME, m (From Reference 27)

Figure 4-12. Volatilization-time Curves for the Photodegradation of Polymethyl Methacrylate and Methyl Methacrylate­Methyl Acrylate Copolymers at 170°c; 0-0-0-PMMA; 0-0-0-MMA/MA ( 112 : 1) ; a-a-11. -MMA/MA ( 26 / 1) ; A-A-~-MMA/MA ( 7 7: 1) ; A-~-A-MMA/MA ( 2 : 1) •

4-17

4'.0

I~ l,:l ,~ 1 i 3.0 0

~ < a.:: C,

~ LI. 0 ~ 2.0 a.:: C, w C

1.0

0.5 1.0 EXPOSURE (HOURS)

1.5 2.0

(From Reference 38)

Figure 4-13. Degree of Degradation vs Exposure at 30°C; 0-0-0-PMMA; t:.-t:.-1:.- (MMA-MA) (1: 1).

4-18

\I') 0 -X

'$ z :::)

~ w ~ 0 z 0 ~ ~ w a.. V,

z 0 (n V,

u V,

~

0 V, ~ z :J V, V,

0 ~ u

10

8

6

4

2

OL... _____ __i ______ ,1__. _____ .....1... _____ ---1.. _____ __.

20 40 60 80 MO.LE% MM.A

(From Reference 26)

Figure 4-14. Dependence of Rates of Scission and Crosslinking on the Composition of MMA/l1A Copolymers Irradiated in the Form of Films

4-19

l Depropagation

!

Copolymer Molecule

lhv

ester group scission

Chain Side Radical

+

Butyraldehyde, Side Chain Fragments

!Chain Scission

Terminal Chain Radical

!oepropagation to nearest Acrylate Unit

Methyl Methacrylate

+

Acrylic Terminated Radical

! l Intermolecular Intermolecular Transfer

i ! 11-butyl acrylate Chain Fragments Chain Radical

! !!_-butyl methacrylate

n-butanol, CO

Scheme 4-3

4-20

l

A. INTRODUCTION

SECTION V

ULTRAVIOLET STABILIZERS

Stabilization of polymers from ultraviolet damage has generated a vast literature which will not be reviewed here. Interested readers may consult references 40-53. We shall concentrate on ultraviolet absorbers (screens) and quenchers which dissipate electronic energy before bond cleavage takes place. Stabilizers may function in any one of the following manners:

(1) UVAbsorbers. These materials function as UV screens and generally absorb light of wavelength less than 400 nm. In order to be effective they should not transfer their excitation energy to the polymer, otherwise they would sensitize photodegradation of the polymer.

(2) UV Quenchers. These are molecules which accept excitation energy from polymeric base units and thus deactivate them. Once the excitation energy is transferred, back-transfer has to be avoided, otherwise the effectiveness of UV quenchers will be reduced. Often the same molecule functions both as an absorber and a quencher. The effectiveness of absorbers and quenchers may be measured in terms of stabilization efficiency which is defined as 1 - ~p/$

0, $p being the

quantum efficiency of degradation of the stabilized polymer, and ~o is the corresponding quantum efficiency of the unstabilized polymer. These materials often have relatively low molecular weight, and they may get leached out when the protected polymer is used outdoors. Hence, these molecules may be suitably derivatized so that they can be copolymer­ized and built into the molecular structure of the polymer.

(3) Radical Scavengers. This type of molecule has easily abstractable hydrogens so that they can react with a propagating radical center and generate a stable radical whose reactivity is too low for it to continue degradation processes.

(4) Antioxidants. There can be several types of antioxidants, e.g., amines, phenols, metal chelates, sulfur containing stabilizers, phosphite esters, etc. The crucial process consists of reaction of the antioxidant molecule with hydro peroxides which are precursors of polymer oxidation.

B. ULTRAVIOLET ABSORBERS AND QUENCHERS

The ultraviolet absorbers and quenchers are potentially the best king of ultraviolet stabilizer, since they seek to activate the elec­tronic excitation energy before it has had an opportunity to cleave any

5-1

bond. The mechanism of their operation is not precisely known and forms an important objective of research being carried out at JPL. Two basic criteria may be proposed which will distinguish an ultraviolet stabilizer from a sensitizer. One is that the lowest excited state of the stabili­zer should have a short lifetime, so that radiationless decay from this state is efficient. The second is rate of electronic energy transfer from this excited state to the polymer molecule should be slow, either because the energy of the stabilizer· excited state is too low, or because steric environment of the stabilizer chromophore makes such energy transfer inefficient. A typical stabilizer or quencher might be modeled as in Scheme 5-1. Often the lifetimes of the excited states of the stabilizer molecules are too short to be measured by nanosecond flash spectroscopy and we have resort to picosecond time scale to observe these transients. Using the twin criteria mentioned above, it is hoped that we can predict stabilization efficiency of any additive, and more importantly, its rate of consumption which is directly related to its lifetime outdoors.

hv 1* (V.1) p )

p

p l* k2 ~ p 3* (V.2)

Q hv ) Q

l* (V. 3)

Q l* k4

) Q 3* (V.4)

p l* ks ) pl

(V.5)

pl*+ Q k6 )' Q l* + p (V.6)

Q 1* k7

) Q 3* (V. 7)

Ql* + p kg

') Q + p l* (V.8)

Q3* + p kg

), Q + p3* (V.9)

p 3* + Q k10 )i Q3* + p (V.10)

Q 3* kll

> Q or Q1 (V.11)

p 3* k12 ") p2

(V.12)

Ql k13

") Q (V.13)

Scheme 5-1

5-2

Here Pis the polymeric base unit, Q is the stabilizer/quencher, and Q1 is a ground state isomer of Q. Ql* is the excited singlet and Q3* is the excited triplet of the stabilizer. P1 and Pz are photo­products, resulting from chain scission or molecular rearrangement. A good UV screen has a high extinction coefficient (V.3), high values of k4, k7, k11, and if applicable, k13. Energy transfer processes (V.10 and V.6) should be rapid, while processes V.8 and V.9 should be slow. In order to function properly, the stabilizer should have zero or near zero quantum yield of photo reaction. One factor affecting the stabili­zation efficiency of the UV screen quenchers not mentioned in the scheme is the solubility of the stabilizer in the polymer. Solubility or miscibility has an important effect in photostabilization of poly­ethylene using various stabilizers. The main classes of ultraviolet stabilizer/quencher are, a) substituted benzophenones40-43 b) nitrogen containing stabilizers41 c) aryl esters (salicylates) and d) aryl acrylates. There is also a group of metal chelates which function as ultraviolet stabilizers the mechanism of action of which is controversial. A hypothesis which is gaining in popularity is that these52 chelates stabilize against photooxidation through quenching of singlet oxygen, as shown in the mechanism below (Scheme 5-2).

hv l* ( V .14)

p :> p

p l* ) p 3* (V .15)

p3* + 30 > p + la ( V .16 ) 2 2

la + R ) R02 (V .17 )

2

la + ML ~ ~ 30 + ML ( V .18 )

2 n 2 n

Scheme 5-2

Here Pis the polymeric chromophore, R is the reactive group on the polymer undergoing photooxidation, and MLn is the metal chelate.

1. Benzophenones as Ultraviolet Stabilizers

It has long been known that 2-hydroxybenzophenone or its deriva­tives have very short excited state lifetimes, presumably because of coupling of the carbonyl with the hydroxyl proton (Scheme 5-3).43

5-3

(d~6~[0r~'Or -[c;Jl'(Df /H,

[ ©J~~ r•FAST- ©i~~ Scheme 5-3

Here we have assumed that the singlet intersystem crosses to the triplet which then undergoes rapid radiationless decay due to the intra­molecular hydrogen bonding. However, it is possible that the triplet is not formed at all, but that the singlet undergoes internal conversion to the ground state through this coupling.45 Lamola and Sharp46 have reported weak phosphorescence from certain derivatives of 2-hydroxy­benzophenones in polar solvents at 77°K. This phosphorescence (lifetime~ 10 nsec) disappears if the solvent used is non-polar. The interpretation is that polar solvents hydrogen bond with the hydroxylic proton, making it more difficult for intramolecular hydrogen bonding to develop, and hence the triplet lifetime is sufficiently increased so that phosphorescence can be observed. On the basis of this result we will assume that it is the triplet which is undergoing decay through intramolecular hydrogen bonding. Sensitization experiments and ultra­fast transient measurements now in progress might also give a clue to this problem. We carried out some nanosecond flash photolysis experi­ments on 2-hydroxybenzophenone and certain derivatives including permasorb MA, to measure the rate processes outlined in Scheme 5-2.

Our results may be summarized by saying that we found that the decay rate was too short to be measured in benzene while it was compar­able to laser half width (1 - lOnsec) in ethanol at room temperature. In benzene we detected a long-lived transient which we attributed to

[ci~r l

> 109 sec -l

6 -1 (2-20) x 10 sec

5-4

~~ c;Jrl0-I

the photoenol which had been postulated to form through rapid decay of the triplet. Here we have a photoenol tantomerism taking place in the triplet state. The triplet photoenol rapidly decays to the ground state and then is converted back to the ketone. When examined in PMMA films, the same process is observed, only the conversion of the encl to the ketone is relatively slow(~ 2000 nsec) (Figure 5-1) an observation in accord with the rigidity of the polymer structure. These findings suggest that if a benzophenone based UV screen/quencher is used, proper attention should be given to the polarity of the polymer substrate as well as the mobility of the polymer chains. The stabilization effi­ciency will drop if the polymer develops strong hydrogen bonds with the stabilizer, e.g., acrylates or urethanes. But this type of interaction is possible only if the polymeric chains are flexible, and so a rigid methacrylate rich polymer might be an ideal substrate for this screen. It is also to be noted that observed stabilization efficiencies of benzophenones is only partly explained by the screening action. They also stabilize through quenching polymer excited states, and if complete UV protection is to be sought, the stabilizer may be put in the cover as well as in the pottant. Another problem we are studying is the stability of the stabilizer itself over long irradiation periods. This stability may be studied by measuring the relative rates of conversion of the enol to the ketone and its other reactions, including its excitation (it absorbs strongly at 550 nm). These rates are being measured using flash spectroscopy and steady state kinetics. When the steady state concentration of the enol in a system under irradiation is determined, we can calculate the rate at which it is oxidized (the enol which is quinonoid, should be susceptible to oxidation) and hence the rate of consumption of the stabilizer.

2. Nitrogen-containing Stabilizers

Hydroxyphenyl benzotriazoles41 have the same type of intramolec­ular hydrogen bonding dapacity as ortho-hydroxybenzophenones, and probably stabilize through the same mechanism, i.e., tautomerism leading to the formation of ketone (Scheme 5-4).

Figure 5-2 shows spectra of some benzotriazoles. Heller41 investigated ortho-hydroxyphenyl s-trizines as stabilizers. The mech­anism of stabilization is similar to that of triazoles.

HYDROXYPHENYL 5-TRIAZINE

5-5

u C) 0 ..J

1.5

1.2

1. 1 MONITORING WAVELENGTH = 350 nm

o.__ _____ ......._ _____ ....L.. _____ __,_ _____ ___.. _____ -.J

0 100 200 300 400 500

-9 TIME x 10 sec

Figure 5-1. First Order Decay of Photoenol Transient Absorption on Excitation of a Solution of 2-Hydroxygenzophenone in Benzene with a Nitrogen Laser (337 nm)

5-6

~ 0

)(

@:N~ M

Ny-R IN CHCl3

~ -)(

N R = - c6H5 (1)

3 - Me 4 - OH c6H5 (2)

2 - OH 5 - Me (3)

b 2 - Me 4 - OH (4)

-)(

280 320 360 nm

(From Reference 41)

Figure 5-2. Absorption Spectra of some Benzotriazoles

0-/o OH

0-NQ q / N .. .. N@ I

\ ~ / ' " N N

OH r l OH N/No oe N

N / / I @,N() ' ~ / N

Scheme 5-4

5-7

The lightfastness and protective power is improved by the number of orthohydroxy groups. The basicity of the s-triazine ring controls its lightfastness and reduced basicity improves photostability.

3. Other Photostabilizers

Salicylates (ortho-hydroxyphenyl acetates) have been successfully demonstrated to have stabilization activity. The mechanism of opera­tion is similar to that of benzophenones, with one important difference: salicylates undergo molecular transformation to ortho-hydroxy benzo­phenones on photolysis (Scheme 5-5).

Scheme 5-5

The photoproducts are also good ultraviolet stabilizers. and absorb at longer wavelength then phenyl salicylate. Therefore, phenyl salicylate is mainly seen as a precursor to the ultimate stabilizer, although it possesses some stabilization properties itself.

Aryl acrylates and acrylonitriles47 have also been used as stabilizers. The mechanism of its action is not known.

C

"-. C N

UVI NUL N-35 OR ETHYL 2-CYANO 3, 3-DIPHENYLACRYLATE

5-8

Organometallic compounds such as ferrocene and ortho hydroxybenzoyl ferrocene have been found to be extremely effective in protecting certain coatings exposed to sunlight.

0

~ ~-~-~ v-Fe-0 H00 0 - HYDROXYBENZOYLFERROCENE

There are several metal chelates notably complexes of nickel, which function as photostabilizers,48,49 although their UV screening efficiency is poor. This activity may be attributed to their quenching efficiency of triplet excited states of polymeric base units or sensi­tizer impurities or efficiency of quenching singlet oxygen. Stabiliza­tion efficiency and singlet oxygen quenching efficiency has been correlated by Zweig and Henrickson35 for a poly (styrene - acrylonitrile - butadiene) copolymer using several nickel and cobalt complexes. Carlsson52 found that nickel thiophenolate quenches ketone sensitized photooxidation, although it is not an efficient triplet quencher. lo2 quenching efficiency increases in the sequence Zn(II)/Hn(II)/Cn(II) << Co(II) < Ni(II) ~ Fe(III) for diisopropyl dithiocarbonate chelates. In general nickel and cobalt complexes are photostable and they behave as long time stabilizers, while Fe(III) complexes decompose and become effective photoinitiators.56

C. ANTIOXIDANTS

Antioxidants prevent thermal or photochemical oxidation of polymers,50 and generally function according to the mechanism shown in Scheme 5-6.

Antioxidants react faster with peroxy radicals ROz than with hydrocarbon radicals R·. A typical antioxidant is 2,6-dit-butyl, 4-methylphenol,51 which forms the corresponding phenoxy radical.

OH o· (Me)

3 C C(Me)

3 (Me) 3

C C(Me) 3

+ R02

• :,.

CH 3 CH 3

5-9

+ ROOH

R· + 0 2 ') R02. (V.19)

R02 • + RH ) ROOH + R· (V.20)

R· + AH ) RH + A· (V.21)

R02• + AH ) ROOH + A· (V.22)

AH+ ROOH Stab 1 e Products (V.23)

A· + R02. Stable Products (V.24)

AH + R02 . ROO- + AW + (V.25)

2A· > Stable Products {V.26)

Scheme 5-6

o· 0 0

Me 3 c, CMe3 CMe 3 ·&CMe3

( l ' . I + ROO. )

' ., OOR

CH 3 CH 3 CH 3

o· OH ~Me 3 CMe

3 CMe 3 '$ CMe 3 \

+ 0 ~ o=Q=CH a· I

CH 3 CH3

CMe 3 + CH 4 CMe3

Scheme 5-7

5-10

Electron withdrawing substituents on the ring decrease antioxidant reactivity while electron donating substituents increase it. Thomas and Foote54 proposed that substituted phenolic antioxidants may react with and quench singlet oxygen. Aromatic amines also possess antioxidant activity, and sometimes this activity is due to their ability to decom­pose peroxy radicals to non-radical (ionic) products. Schultz,53 et al., reported that gramine type compounds and amino pyrazoles are effective antioxidants. These antioxidants may function as chain terminators as well as peroxide decomposers, since the aromatic aza compounds can stabilize radical cations. Hawkins has reported several examples of synergism between chain terminator and peroxide decomposer antioxidants. The origin of synergism may be that the chain terminator might reduce the oxidative chain length and hence the hydroperoxide concentration, so that the effectiveness of the peroxide decomposer is enhanced. Fig-ure 5-3 demonstrates this phenomenon in polyethylene. Sometimes an antagonistic effect is observed, for example, when phosphites are used with phenols.57 Phosphites show strong synergism when used with UV absorbers and quenchers such as hydroxy benzophenones and hydroxy benzotriazoles.58 As in the case of UV stabilizers or quenchers the effectiveness of any antioxidant depends on compatibility (physical and chemical) with the polymer, stability of the antioxidant at processing temperatures, rate of leaching of the antioxidant from the polymer, etc. These factors have to be taken into account when selecting an antioxidant for a particular polymer formulation.

CH2

((]CR I 1 H

C2H502C NH-©- R

NHD 2 I

H

Gramine phenylami'nopyrazole

5-11

100

~ 80 u

~s-s~ a)~ ~0.1%

~ a) ~ ~ - 60

< w ~ <(

t: 40 :::,

z w C)

~ 20 0

b)

c) 0. 1 % OF a PLUS O. 1 % OF b

c)

0 Lo.a=;:;;;;!~100~;;~===2ot:o=-=l::===3:too~~==:400r::=~----sooj HR_.

(From Reference 56)

Figure 5-3. Synergism between a Disulfide and a Phenolic Antioxidant in Polyethylene

5-12

SECTION VI

CONCLUSION

In conclusion, it would.be appropriate to re-emphasize our basic objective: be able to predict the lifetime of encapsulated solar cell modules in terms of cell power output vs time after twenty years or more of outdoor use. The foregoing discussion demonstrates that there are essentially three levels of testing and data analysis on modules which pass the short time quality assurance tests, namely, mechanism of photo and thermal degradation of encapsulant materials, relationship between these chemical changes to physical properties which affect per­formance of the encapsulant, e.g., loss of modulus of the cover result­ing in increased dirt retention, loss of UV screen from the cover resulting- in insolation of pottant underneath, change of surface energy resulting in delamination, etc., and the relationship between change in physical properties and change in cell performance. Sometimes the structure of these links will be more complex, since, a) one type of chemical change will affect more than one physical property, and so "cross terms" will be important, and b) it may not be possible to directly relate a performance related physical property to chemical structure without using intermediate links, e.g., glass transition temperature can be related to molecular weight distribution which in turn could be related to chain scission, development of branches and crosslinking.

One conclusion we can draw from the material presented here is that some mechanistic interpretations are directly applicable to polymer formulation to serve a certain encapsulation function. For example, the experiment carried out by Grassie on.poly methyl methacrylate -methyl acrylate copolymers, if repeated at temperatures likely to be encountered by solar cell modules, should indicate the composition of the most stable acrylic copolymer. With due concern to other desirable physical properties such as low Tg, low permeability to harmful vapors, good adhesion to cells and substrate, or an adhesive (primer) to be used, we can then propose an acrylic copolymer which could function as a durable, low cost pottant. Mechanistic studies can predict the lifetime of this copolymer in any environment if it is accurately monitored. On the basis of this determination it is possible to predict whether a low cost pottant is going to be durable for twenty years or more outdoors, or if it is necessary to stabilize it by adding stabili­zers. We can then predict what stabilizer would be most effective and long lasting in a certain system. Thus, from the viewpoint of predic­tion of deterioration of cell performance due to change of chemical structure of the encapsulant it appears that the focus should be on the pottant. The function of the cover should then be to provide protection from ultraviolet, oxygen moisture, dirt, pollutants, and so on. The degree of protection desired will depend on the sensitivity of the pottant to degradation and the lifetime reliability desired. In addition, in certain designs the cover is designed to bear wind load and hail impact. Hence, the cover cannot be selected without choosing

6-1

the pottant and making some cost/quality tradeoffs. Similarly, the primer and the substrate cannot be chosen without regard to the chemical structure of the pottant. This is why in our inhouse work we have placed early emphasis on potential pottants, and when mechanistic models of significant material choices are constructed, we hope to have a clear picture of what kind of protection is needed from the cover and the substrate. Ranking and screening work by Springborn Labs and Battelle has already uncovered several types of pottant materials, differing in cost and weatherability, in addition to other properties sought for in pottants. Some of these materials will be studied in some detail both by us and by contractors.

6-2·

REFERENCES

1. Coulbert, C., Development and Validation of a Life-Prediction Methodology for LSSA Encapsulated Modules, LSSA Report No. 5101-40, June 1977.

2. Duncan, C.H., Hobbs, A. J., Pai, M. S., Spectral Radiances of Some High Intensity Light Sources, NASA Report No. X663-63-17.

3. Hawkins, W. L., Polymer Stabilization, W. L. Hawkins, ed., Wiley-Interscience, New York, 1971; p. 414.

4. Handbook of Geophysics and Space Environments, s. L. Valley, ed., McGraw Hill, New York 1965; p. 16-2.

5. Gupta, A., Unpublished Results. JPL, Pasadena, Calif.

6. Data Sheet from the Canrad Precision Industries, Inc., Hanovia Lamp Division, New Jersey.

7. Moses, M. G., Liu, R. S. H., and Monroe, B. M., Journal of Molecular Photochemistry, 1969, p. 245.

8. Gupta, A., and Hefter, H., Unpublished Results, JPL, Pasadena, Calif.

9. Murov, S. L., Handbook of. Photochemistry, Marcel Dekkar, 1973; p. 99

10. Murov, S. L., Handbook of Photochemistry, Marcel Dekkar, New York 1973; p. 103.

11. Ultraviolet Radiation in Los Angeles, 1965, U. S. Department of HEW, Public Health Service, p. 21.

12. Nader, J. S., Smith, C. F., ibid., p. 5.

13. Arveson, J. C., Applied Optics,~, 2215 (1969).

14. Johnson, F. S., Journal of Meteorology, 11, 6, (1954).

15. Nicolet

16. Hatchard, C. G., Parker, C. A., Proceedings of the Royal Society (London) A 235, 518 (1956).

17. Calvert, J. G., and Pitts, J. N., Jr., Photochemistry, Wiley, New York 1966; p. 783.

18. Wagner, P. J., and Capen, G., Journal of Molecular Photochemistry, .!.., 173 (1969).

6-3

19. Pitts, J. N., Jr., Wan, J. K. S., and Schuck, E. A., Journal of American Chemical Society, 86, 3606 (1964).

20. Springborn Laboratories, 6th Quarterly Progress Report, ERDA/JPL 9 5 4 5 2 7 , ( 19 77) ; p • 5-2 .

21. Smith, T., Rockwell Science Center, Thousand Oaks," Annual Progress Report, DOE/JPL 954739 (1978).

22. Rainhart, L. G. and Schinunel, W. P., Solar Energy 17, 259, (1975).

23. Isakson, K. E., Journal of Paint Technology, 44, 41 (1972).

24. Reinisch, R. F., and Gloria, H. R., Polymer Preprints, American Chemical Society, Polymer Chemistry Division, -2_, 349 (1968).

25. Golemba, F. J. and Guillet, J. E., Journal of Paint Technology, 41, 315 (1969).

26. Grassie, N., Pure and Applied Chemistry 34, 247 (1973).

27. Grassie, N., Torrance, B. J. D., and Colford, J. G., Journal of Polymer Science Part A-1, ]_, 1425 (1969).

28. Maccullum, J. R., and Schoff, C. K., Transactions of Faraday Society~ ,Zl, 2383 (1971).

29. Fox, R. B., Isaccs, L. G., and Stokes, S., Journal of Polymer Science, A-1, ..!_, 1079 (1963).

30. Fox, R. B., Isaacs, L. G., and Stokes, S., U.S. Naval Research Laboratory Report No. 5720 (1971).

31. Cowley, P. R. E. J., and Helville, H. W., Proceedings of the Royal Society A211, 320 (1952).

32. Charlesby, A., and Thomas, D. K., Proceedings of the Royal Society (London), A269, 104 (1962).

33. Frolova, M. I. , Ef imov, L. I., and Riabov, A. V., J. Khim i Khim Teknol. ]_, 304 (1964).

34. Morimoto, K., and Suzuki, S., Journal of Applied Polymer Science, 16, 2947 (1972).

35. Zewig, A., and Henderson, W. A., Journal of Polymer Science, 13, 993 (1975).

36. Fox, R. B., Isaccs, L. G., Stokes, S., and Kagarise, R. E., U.S. Naval Research Laboratory Report, No. 5730 (1961).

37. Fox, R. B., Isaccs, L. G., Stokes, S., and Kagarise, R. E., Journal of Polymer Science A-1, 1, 2085 (1964).

6-4

38. Allison, J. P., Journal of Polymer Science A-1, !t_, 1209 (1966).

39. Morimoto, A., Takamitsu, I., Progress in Organic Coatings,.!, 35 (1973).

40. Gordon, D. A., Encyclopedia of Chemical Technology Vol. 21, 2nd ed., John Wiley, New York 1970; p. 115.

41. Heller, H. J., European Polymer Journal, Supplement 105 (1969).

42. Newland, G. C., Tamblyn, J. W., Journal of Applied Polymer Science,~' 1949 (1964).

43. O'Connell, E. J., Journal of American Chemical Society, 90, 6550 (1968).

44. Ranby, B., and Rabek, J. F., Photodegradation, Photooxidation and Photostabilization of Polymers, Principles and Applications, Wiley-Interscience, New York 1975. Ch. 10.

45. Klopffer, W., Degradation and Stabilization of Polyolefins, ed. B. Sedlacek, et al., John Wiley, New York 1976; pp. 205.

46. Lamola, A. A., and Sharp, L., Journal of Physical Chemistry, 70, 2634 (1966).

47. Lappin, G. R., Encyclopedia of Polymer Science and Technology, Wiley-Interscience, New York 1971; vol. 14, p. 125.

48. Briggs, P. J., and McKellar, J. F., Journal of Applied Polymer Science, 12, 1821 (1968).

49. Hammond, G. S., and Foss, R. P., Journal of Physical Chemistry, 68, 3747 (1964).

50. Scott, G., European Polymer Journal, Supplement, 189 (1969).

51. Scott, G., British Polymer Journal l, 24 (1971).

52. Carlsson, D. J., Sproule, D. E., and Wiles, D. M., Macromolecules, 5, 659 (1972).

53. Schultz, M., Wegart, W. H., Stampehl, G., and Riediger, W., John Wiley, 1976; pp. 329.

54. Thomas, M., and Foote, C. S., in Ranby, B., and Rabek, J. R., Ref. 44.

55. Bouzer, C. E. and Hammond, G. S., J. Amer. Chem. Soc., J.j_, 3861 (1954).

6-5

56. Hawkins, W. L., Degradation and Stabilization of Polyolefins, ed., B. Sedlacek, et al., John Wiley, New York 1976; pp. 319.

57. Maassen, G. C., Fawcett, R. J., and Connel, W.R., Encyclopedia of Polymer Science and Technology, Wiley-Interscience, 1965; vol. 2, p. 171.

58. Fitton, S. L., Haward, R. N., and Williamson, G. R., British Polymer Journal, 1, 217 (1970).

6-6