Polymers in Solar Energy Utilization - American Chemical Society

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New Approach to the Prediction of Photooxidation of Plastics in Solar Applications J. E . GUILLET, A . C. SOMERSALL, and J. W. GORDON University of Toronto, Department of Chemistry, Toronto, M5S 1A1 Canada

A critical feature of the designs for solar arrays is that the components must maintain their integrity and function for an extended period of time in an outdoor environment. There is little experience on plastic cover and capsulant materials which could be used to predict the long-term stability and performance of these materials, and no scientific basis on which one could make reliable extended lifetime predictions. We have, therefore, developed a computer model which can generate realistic concentration versus time profiles of all chemical species formed by photooxidation using as input data a choice set of elementary reactions with corresponding rates and initial conditions. Attempts are being made to interpret some potential failure mechanisms in the solar applications in terms of predicted chemical changes such as scission, crosslinking and oxidation over time. Photooxidation of an ethylene-vinyl acetate copolymer was examined as a particular case. Any attempt to develop a technology for producing low-cost, longlife photovoltaic modules and arrays must come to terms with the weathering effects experienced by the materials exposed to the sun s ultraviolet, oxygen, moisture and the stresses imposed by continuous thermal cycling, among other things. Polymeric substances which could find application as convenient protective covers, pottants/adhesives and backcovers undergo slow, complex photooxidation which changes the chemical and physical properties of these materials. Absorption of the ultraviolet in the tail of the solar spectrum can cause the breaking of chemical bonds, resulting in embrittlement and increased permeability, or to crosslinking which can produce shrinking and cracks. In addition, oxidation often leads to discoloration and reduced transparency, and to the formation of polar groups which could affect f

0097-6156/83/0220-0217$06.00/0 © 1983 American Chemical Society Gebelein et al.; Polymers in Solar Energy Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

218

POLYMERS

IN SOLAR

ENERGY

UTILIZATION

electrical properties. These changes obviously l i m i t the usefulness of solar devices which, for economic reasons, must p e r f o r m satisfactorily for up to 20 years in rather severe environments. Much work has already been done to unravel the complexities of photooxidative processes and to develop some highly effective light (UV) stabilizers which can delay the onset of polymer embrittlement. A good review has been presented by C a r l s s o n et al.(l). Some polymer s y s tems have also been exploited to fabricate plastics with controlled l i f e times in the short range(2,3). However, very little is known about the ultimate changes that occur in polymeric substances after very long periods such as the 20-year regime appropriate for economic photovoltaic power plants. There is no good way to predict the rates of the chemical and/or physical changes which occur from accelerated tests. In part, the p r o b lem has been that there is no adequate laboratory method to measure these effects over such extended times. Furthermore, accelerated tests (in which materials are exposed to higher intensity U V sources in controlled atmospheres) are of limited value in predicting rates since there is often no reciprocity between intensity and time of exposure. A n alternative approach developed in our laboratories has been to simulate the process of photooxidation with a computer model which could be verified with experimental data from accelerated and outdoor expos u r e s . Extrapolation of the numerical solutions for species concentrations over time should provide some understanding and control of the degradation process over extended lifetimes. Computer Model The model consists of a set of reactions (Table I) for the basic r e action sequence based on the now well established mechanism of hydrocarbon oxidation. Rate parameters have been assigned to these fundamental equations, based on our best estimate from the literature( 4) (Table I ) . A problem with such a simulation study is that the predicted rates w i l l be only as reliable as the rate parameter data base employed. We anticipate continuous refinement of this data base in further w o r k . In addition, a number of techniques has been developed for performing the sensitivity analysis required to appreciate the relative importance of particular reactions and rate data( 5 ) . Numerical methods for the solution of the resulting large set of coupled differential equations have been developed. The method o r i g i nally due to Gear( 6) has been applied by A l l a r a and Edelson for the p y r o l y s i s of alkanes(7) and later by others( 8,9) for similar p r o c e s s e s . Smog formation and detailed small-molecule photochemistry have also been studied by these numerical simuIations(lO). Semi-quantitative

Gebelein et al.; Polymers in Solar Energy Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

14.

GUILLET

Prediction of Photooxidation of Plastics

ET AL.

Table I.

219

Elementary Reactions in Polymer Photooxidation and Corresponding R a t e s - ' Reaction

RO

rt

2

+ RH

—>

Rate constant

ROOH

0.1 x 1 0 "

+RO„

2

2

—>

R O H + Ketone + *0

0.2

R O . + ROH Δ

—>

ROOH + Ketone + HOO

0.5 χ 10"

1

HOO + R H

—>

HOOH +RO

0.5 χ 10"

2

RO

+ RO

rt

2

ft

2

χ 10

8

Δ HOO + R O

—>

ROOH

+

—>

ROOH

+ Iteroxy C O

0.5 χ 1 0 "

2

—>

ROOH

+ Ketone + O H

0.5 χ ΙΟ"

1

—>

ROOH

+ Aldehyde + HOO

0.5 χ 10"

2

—>

ROOH

+

0.1

χ 10

3

—>

R0

0.3

χ 10

9

KET*

0.3

χ 10

KET*

0.3

χ 10"

X

Δ R0

R0 RO R0

2

+ Ketone •

2

2

ft

2

+ ROOH

+ SMROH •

+ Aldehyde OH

+ RH Ketone

0.1

SMRCO

+ Water

χ 10

1 0

-5

hv

SMKetone

>

SMR0

2

+

SMRCO

>

SMR0

2

+ CO

KET*

>

Alkene + SMKetone

8 0 . 5 χ 10

2

>

Ketone +

0.1

χ 10

+ ROOH

>

Ketone + RO + OH

0.1

χ 10

KET*

>

Ketone

0.1

χ 10

0.6

χ 10

+

0

SMRCO

1

Ο

Λ

5

7 0 . 5 χ 10

KET*

KET* KET*

2

0. Δ

6 0 . 5 χ 10

1 0

2

>

8

1 0

5

°2

Continued on next page


220

POLYMERS

IN SOLAR

ENERGY

UTILIZATION

Table I (continued) Reaction

1

Rate constant

->

ROOH

+ RH

->

SMROOH

SMROOH

->

SMRO + R H

0

+ Alkene

SMR0

2

SMRCO

+ O. 2

SMRCOOO

+ RH

SMRCOOOH ROOH

hv

0.1 χ 1 0

4

0.1 χ 10"

2

SMRO + OH

0.3 χ 10"

4

->

SMROH

0.1 χ 1 0

->

SMRCOOO

0.4 χ 1 0

->

SMRCOOOH +

->

SMRC0

hv

+ RO

rt

2

R0

2

+ OH

2

0.1 χ 10

RO + OH

0.3 χ 10

RO

SMR0

0.1 χ 10

RO + R H

->

RO

+ Aldehyde

+ ROH

1 0

0.1 χ Ι Ο "

-> ->

2

6

1

-3 -4 6

0.1 χ 10

6

0.1 χ 1 0

6

0.1 χ 1 0

5

il

SMRCO.

+ RH

->

Acid +

->

ROOR

R0

2

u

RO 2

+ RO 2

a -3 - [0 1 =10 M (constant); SMProduct = Product from chain cleavage. 2

- T h i s set of reactions and rate constant values is by no means final. We have used it only to demonstrate the approach and to test the use fulness of the method by adjusting a few of the important variables. Much more work is required to identify the most appropriate input data block.


14.

GUILLET

ET AL.


221

prediction has been possible in these c a s e s . However, we report here the first attempts to simulate photooxidation kinetics in the case of p o l y m e r s . Our modified program calculates by stepwise integration in r e a l time the varying concentrations of chemical species formed during photooxidation. T o validate our numerical procedure we employed the data base given for the cesium flare system and generated curves identical to those of Edelson(ll) for the same system (Figure 1 ) . The excellent agreement between predicted and actual rate curves shows that the p r o gram itself (irrespective of the data base) performs in a satisfactory manner. Mechanism of Photooxidation A s a starting point for polymer photooxidation we looked at a f o r mal linear low-molecular-weight alkane (RH). In principle, this should be similar to amorphous high density polyethylene where short-range diffusion rates in reaction centres should approach that of viscous liquids In practice, many polymers w i l l show only chemical changes in the hydro carbon moiety since bond breakage w i l l commonly take place initially in the more labile C - H and C ~ C bonds. Initiation w i l l take place following U V absorption by ketone or hydroperoxide groups or even fortuitously by some C - H bond cleavage. The possibilities of energy transfer among different groups have also been included in the model. Propagation takes place via the formation of peroxy radicals followed by hydrogen atom abstraction from the backbone and repeated fast oxygen addition. Peroxy radical chain c a r r i e r s terminate by disproportionation to form alcohols and ketones. Further photolysis of ketone products leads to another autocatalytic chain. Initiation

RH

?

-> R -

+o

2

fast

->

R 0

2

-

Ketone

10 ROOH

-10


222

POLYMERS

IN SOLAR

ENERGY

UTILIZATION

Ο

L o g time, s Figure 1. Our numerical solutions for concentration versus time p r o files for the cesium fiare (cf. r é f . 10): ( Ο ) Ν · ( · ) Ο^, ( • ) C s , ( • ) C s 0 , ( Δ ) C s , e", ( Α ) Ο". +

2


14.

GUILLET E T AL.

223

Prediction of Photooxidation of Plastics Propagation

Peroxide chain + RH ~~

27

-> k = l(T

ROOH

ROOH

+

OH-

R

H k

„

1

->

>

9

0

+ R-

3

->

fast

RO

RO- + OH-

R

HOH

Ketone chain _ , Ketone

hv

^ >

N o r r i s h II ^ , , . — > SMKetone + Alkene A

KET*

n

0

(ρ — υ .δ k - 1θ8 Norrish I φ ~ 0.02 V SMR-

V 5

KET *

+ SMRCO-

Termination RO 2

η—>

· + RO · 2

RO

k~10

· + RO · 2

2

R O H + Ketone +

7

τ — > ROOR k~10

4

+

Χ

Ο

*0 2 0

Λ

2

ROOH ROo · + 2

' " Ketone Aldehyde

5 =-> k~10~ -10~ 2

ROOH

+ Other products

3

etc. P r e l i m i n a r y Results E a r l y computer modelling results have shown that the processes of photooxidation under the conditions of our scheme would involve a long induction period, of up to several y e a r s in the pure hydrocarbon, f o l lowed by a fairly rapid deterioration (Figure 2). Initiation is effected


224

POLYMERS

Figure 2.

IN SOLAR

ENERGY

UTILIZATION

Photooxidation of linear alkane (RH) versus time.


14.

GUILLET

ETAL.


225

fortuitously in the program by assigning a low rate constant for R - H cleavage or using low initial concentrations (ca. 10" M ) for either k e tone or hydroperoxide moieties. The principal products of photooxida tion are ketones, alcohols, water and alkenes, with smaller quantities of aldehydes, acids, carbon monoxide, etc. (Table Π ) . 5

The rates of formation of the major species show exponential be havior after the induction period i s over (Figure 3 ) . T h i s conforms to the rate behavior for polyethylene observed experimentally in a c c e l e r ated tests (Figure 4 ) . A n increase in the light intensity (reflected in a chosen systematic increase in all the photochemical absorption rates in the program) shows a systematic change in the exponential formation of ketone products (Figure 5) while the induction p e r i o d is shortened ( F i g ure 6 ) . Increase in termination rate shortens the kinetic chain length and reduces the formation of product ketone. In this case, the exponen tial formation of ketone remains the same but is shifted in time to lengthen the induction period ( Figure 7). The autocatalytic process is initiated by a narrow concentration range of ketone initiator. A t low levels ( 10" M) the mutual termination of peroxy radicals is too fast to p e r m i t s i g nificant Η - a b s t r a c t i o n from the substrate. These preliminary results are consistent with a l l of our experience to date on the photooxidation of ketone-containing polymers( 12). 7

3

Conclusions Much work remains to be done in refinement of the model to allow for the inclusion of substituent groups, the reactivity of secondary and tertiary C - H bonds, the significance of diffusion, the influences of tem perature cycling and dark reactions, and the impact of additives. In con clusion, we believe that these modelling studies which can simulate r e a l systems (given reliable input data), represent a novel approach to the general understanding of polymer photooxidation phenomena which should lead to a new understanding of the study of controlled lifetimes for p o l y m e r s and for the development of procedures which would allow the p r e diction of performance of plastics for solar applications.


226

POLYMERS

IN SOLAR

ENERGY

UTILIZATION

Table II. Final Concentration A r r a y . Time of Photooxidation, 10 Y e a r s

Species label

Initial cone, M

Final c o n e . , M

—

0.25 χ 1 0 "

RH

5.0

3.8

ROOH

— —

0.77 χ 1 0 "

R0

2

ROH

10 "

Ketone

^2 HOO HOOH Peroxy C O OH SMROH Aldehyde SMRCO

H 0 2

KET*

— — — — — — — — — —

7

4

0.49 5

0.29 1 2

0.23 χ 1 0 "

1 0

0.25 χ 1 0 " 0.21 χ 1 0 "

4

0.89 χ 1 0 "

3

1 6

0.10 χ 1 0 " 0.19 3

0.19 χ 10""

1 4

0.10 χ 1 0 " 0.40

1 5

0.88 χ 1 0 " 0.14 χ 1 0 "

1

0.14 χ 1 0 "

5

0.19 χ 1 0 "

1

SMRCOOOH

— — — — — — — — —

SMRC0

--

0.11 χ 1 0 "

—

0.15

SMKetone

SMR0

2

CO Alkene ROOR RO SMROOH SMRO SMRCOOO

Acid

2

0.16 χ 1 0

1

0.22 χ 1 0 "

3

1 4

0.48 χ 1 0 " 0.18 χ 1 0 "

3

0.14 χ 1 0 ~

1 3

0.11 χ 1 0 "

6

0.43 χ 1 0 "

4


1 3

14.

GUILLET


ET AL.

227

10 years

I ° 0

1

0.6

1 1.2

1

1

\ 1

1.8 2 . 4 3 . 0

1

Q

Time (x!0 s) Figure 3 . Variation of major product concentrations during photooxida tion: ( Δ ) alkene, (Q) R O H , ( • ) ketone.

Τ

0

20

40

60

Accelerometer, Figure 4.

80

100

hr

Photooxidation of polyethylene.


228

POLYMERS

Time

Figure 5.

ENERGY

UTILIZATION

(xlO°s)

Effect of intensity on product formation during photooxidation: 8

(O) N x 5 , slope 5.5 χ 1 θ " , 1.1x10

8

(•)

g

slope

IN SOLAR

Ν χ 2, slope 2.4 χ 10~ ,

(Δ) Ν,

.

I

0.6

I

I

I

1—

1.2

1.8

2.4

3.0

8

Time(xl0 s)

Figure 6.

Photooxidation as a function of intensity of light:

(•)

(Δ) Ν χ 2, ( Ο ) Ν .


Ν χ 5,

14.

GUILLET

ET AL.


229

Figure 7. Effect of termination rate on product formation during photo oxidation: ( Δ ) D , (O) D x 2 , ( • ) D x 5 .


POLYMERS IN SOLAR ENERGY UTILIZATION

230

Acknovle dgment s T h i s r e s e a r c h is supported by a grant from the Jet Propulsion Laboratory,

Pasadena, California, and is part of a Solar Energy P r o

ject, administered through the U . S . National Aeronautics and Space A dministration.

Literature Cited 1.

2. 3. 4.

5. 6. 7. 8. 9. 10. 11. 12.

Carlsson, D. J.; Garton, Α . ; Wiles, D. M . "Developments in Polymer Stabilisation", vol. 1, G . Scott, Ed., Applied Science, Barking, 1980. Guillet, J. E . "Polymers and Ecological Problems", J. Guillet, Ed., Plenum, New York, 1973. Scott, G . J. Polym. Sci., Polym. Symposia 1976, 57, 357. We acknowledge useful discussions on this subject with Dr. Keith Ingold of the National Research Council of Canada. Any omissions or errors are ours. Edelson, D.; Allara, D. L . Int. J. Chem. Kinet. 1980, 12, 605. Gear, C. W. Commun. ACM 1971, 14, 176. Allara, D. L.; Edelson, D. Int. J. Chem. Kinet. 1975, 7, 479. Sundaram, K . M . ; Froment, G . F . , Ind. Eng. Chem. Fundamen. 1978, 17(3), 174. Olson, D. B . ; Tanzawa, T . ; Gardiner, W. C . J r . Int. J. Chem. Kinet. 1979, 11, 23. Ebert, K . H . ; Ederer, H. J.; Isbarn, G . Angew. Chem. 1980, 19, 333. Edelson, D. J. Chem. Ed. 1975, 52, 642. Guillet, J. E. Pure Appl. Chem. 1980, 52, 285.

RECEIVED February

18, 1983


Polymers in Solar Energy Utilization - American Chemical Society

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