A computer program developed to model and calculate by numerical integration the varying concentrations of chemical species formed during photooxidation of a polymeric material over time, using as input data a choice set of elementary reactions, corresponding rate constants and a convenient set of starting conditions is evaluated. Attempts were made to validate the proposed mechanism by experimentally monitoring the photooxidation products of small liquid alkane which are useful starting models for ethylene segments of polymers like EVA. The model system proved in appropriate for the intended purposes. Another validation model is recommended

Guillet, J. E.

Somersall, A. C.

English

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July 1981
Modelling of Polymer Photodegradation
for Solar Cell Modules
A Quarterly Technical Progress Report
Covering the Period April i - June 30, 1981
(NASA-CR-165040) MODELLING OF POLYMER
	 N82-14,358PHOTODEGRADATION FOR SOLAR CELL MODULESQuaritecly Technical p rogress .Report, 1 Apr.
- ^30 Jun. 1981 (Toronto Univ.) 16 p
	 UnclasHC (A02 /MF A01
	 CSCL 07C G3/27 08483
A. C. Somersall and J. E. Guil et*
r
Principal Investigator
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Department of Chemistry, University of Toronto V
Toronto, ,Canada M5S tA1
LSA Project, Technology Development Area
JPL 955591 --- 2
Contract Goala and Objectives
As part of the Encapsulation Tasks this research prof ram is intended
tc , model the photodegradation of synthetic polymers used as pottants and/or
oaver sheets in the LSA solar cell module designs. It involves the develop-
meat of a computer simulation of the chemical processes that take place
unde :°
 weathering conditions Which could, in principle, relate directly to the
performance of these materials and afford some basis for predicting and/3r
controlling their useful lifetimes.
The program can be divided into three main parts;
I. The development of a computer program to model the weathering/
hhotooxidation of an ethylene-vinyl acetate copolymer as a typical
candidate for LSA applications.
2. The development of new analytical procedures for the determination
of photooxidation and photodegradation at early stages in solid poly-
mer samples.
3. The development of weathering tests suitable for use with a computer
kinetic model to provide a basis for extrapolated predictions.
"This report was prepared as an account of work sponsored by the United States
Covernment. Neither the United States nor the United States Department o£ Energy,
nor any of their employees, nor any of their contractors, subcontractors, or their
employees, makes any warranty, express or implied, or assumes any legal liability
or responsibility for the accuracy, completeness or usefulness of any information,
apparatus, product or process disclosed, or represents that it3 use would not
infringe privately owned rights."
3Summary
JPL 955591 - 3
S
A computer program has been developed to calculate by numerical
Integration the varv►ng concentrations of chemical species formed during
photoo.ddation of a polymeric material over time, using as Input data a
choice set of elementary reactions, corresponding rate constants and a con-
venient set of starting conditions. Early attempts have been made to validate
the proposed mechanism by experimentally monitoring the photpoxldation
products of small liquid alkanes which are useful starting models for ethylene
segments of polymers
	
EVA .
During the second quarter of this year, there have been some major
developments in our research program.
1.	 The Model
In earlier computer simulations we found that using our best set of
elementary reactions and rates, the calculation would proceed through as
many as 1000 Iteration steps without any signifteant photooxidation of the
alkane (RH) taking place. We therefore arbitrarf I,y increased the input rate
constants for chain propagation ' oaction .
 3) by several orders of magnitude
and then we could observe extensive chemical change In the hydrocarbon:
RH > R • (1)
Initiation
R • •* OZ >ROZ• (2)
RO2 • + RH > RO 2H + R •	 Propagation (3)
ROZ • + RO„ • > Alcohols + ketones (4)
Termination
F,^02H > Alcohols +' (5)
E.Verimentally, we also observed very low yields of the isomeric ketones
and alcohols. These tended to level off with time, indicating the absence
of any significant chain mechanism. Following consultations with Dr. Keith
x
is	
JPL 955591 -- 4
insold of the National Research Coup.a l of Canada, we now appreciate that at
room 'temperature tho rate of reaction (3) is so slow Indeed that the termina-
tion step (reaction 4) dominates the photcoxidation. Despite the ambiguity
of reactions (4) and (5) predicting the same observed products, we conclude
in effect, that the products we have been separating during the photooxidation
of the liquid alkanes have been the products of photooxidation of the ketone
and/or peroxide Initiators, with m ►nimal alkane solvent derivatives formed
at best in extremely short chain lengths. It is not surprising then that
such high concentrations (-ap to 10%) of initiator were necessary to observe
significant amounts of product In days of irradiation with a medium pressure
mercury lamp. The absorption of the initia tors is very low at 313 iun so the
product yields were correspondingly low, since no chain process was effec-
tively initiated.
We therefore conclude that this model system has now proved Inap-
propriate for our purposes and intend to use a different one in the future. A
much better model system to validate any simulation of photooxidattion aright
be the Ecolyte system. This is apatented formulation developed In Professor
Guillet's laboratories, constoting of mixtures of smatll amounts of ketone .-
containing polymers blended with other commercial polymers in such a way
as to afford prograunmed lifetimes for the restdting materials which we now
can attempt to simulate.
2.	 The Method
Another major problem we have been facing is the short oxidation
times observed in the computer output. Typi , ,al results showed oxidation In
a few days at most. When we reduced the propagation rate to lengthen the
lifetime of the atlkane, the integration procedure was completed without ally
significant oxidation. Yet, what we really would litre to generate is behaviour
over 10 to 20 years. This raised the question of the adequacy of the method;
could the p-xogrwn really integrate over several years or is there an inheront
limitation (such as the accuracy limitation to the maximmu si7.e of the step
length) ? The methorl has now been shown to be adequate.
1
JPL 955591 - 5
We have taken two steps. First, we have completely revised the rile-
mentary reaction matrix, with some consultation from Dr. Ingold again,
resulting in a uew scheme of 31 or 32 reactions (see Appendix) . This set
Is somewhat incomplete for EVA since no allowance Is made for the dif-
fe,"ent secondary and tertiary C-H groups and no corrections are included
for the restricted. diffusion of polymeric radicals in the solid state. Secondly,
we have gained operational control of our integration parameters to increase
the flexibility with the program.
The result is that without any arbitrar^ ,-r adjustment to the reaction
rates, the computer simulation now shows for the first time lifetimes in
excess of ten years when initiated by very small amounts of ketone, peroxide
or some fortuitous alkyl radical-generating step (see Appendix) .
Our results to date lead us to some tentative observations.
	
1.	 The photooxidation process has a long induction period of up to several
years, followed by a rapid deterioration.
	
2.	 The chemical changes that result lead to
(a) The formation of alcohols with polar'-OH groups which could affect
wettability and electrical resistance of the polymers.
(b) The formation of ketones which could become conjugated, leading
to discoloration and which could also lead to scission and Ginali
molecule fragments, and
(c) Crosslinking through alkoxy radicals in a cage couplin gCS  to form
peroxide linkages.
RO +RO • > R-0-0— R	 (6)
This crosslinking could cause the polymer to shrink and hence
tear or pull away from connections and supports in a module
failure mode.
	
3.	 The most appropriate stabilioation mechanism would probably be an
efficient peroxy radical trap since it appears that the peroxy radicals
dominate the chemistry.
Appendix
Some Examples of Input-Output Data
Used in the Current Simulation Routine
for Alkane Photooxidation
N.B. A new plotting subroutine has been interfaced to provide more
convenient graphs for the output of the concentration-time profiles.
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Modelling of polymer photodegradation for solar cell modules

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