A computer model including an integration routine was developed and demonstrated to simulate, in principle, the chemical changes which may occur in the photooxidation of hydrocarbons, using as input data a set of elementary reactions, corresponding rate constants and appropriate starting conditions. Application of this model to the photooxidation of pottant and plastic materials used in the LSA module designs provides a reliable predictive capability regarding the useful lifetime of these materials. An earlier mechanism consisting of 46 reactions was simplified considerably by reducing the number of formal termination steps since it became apparent that the major termination process goes via the peroxy radicals. In addition, new reactions of oxygen with acryl radicals (from Norrish type I) to form peracids, which then decompose to form carbon dioxide are included

Guillet, J. E.

Somersall, A. C.

English

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https://ntrs.nasa.gov/search.jsp?R=19820006484 2020-03-21T10:05:30+00:00Z
995o^-^>s
JPL 955591- I^Z
April 1081
Modelling of Polymer Photodegradation
for Solar Cell Modules
A Quarterly Technical Progress Report
Covering the period January 1 - March 81 0 1981
A. C . Somersall and J . E . Guillet*
* Principal Investigator
p")i 
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	^'^.l^'^pUl
IRV. NCO
4^^`Y 
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Department of Chemistry, University of Toronto
Toronto, Canada M5S 1A I
(NASA-CH-1 504"1) MODELLING OF POLYMER
	 N82-14357PHOTODEGRADA'TION FOR SOLAR CELL MODULES
Quarterly Technical Progress report, 1 Jan.
- 31 Mar. 1981 (Toronto Uviv.) 17 p
	 UnclasHC A02/MF A01
	 CSCL 07C G3/27 08482
LSA Project, Technology Development Area
I
, I
JPL 955591 -- 2
Contract; Goals and Objectives
ti
As part of the Encapsulation Task, this research program is Intended.
to model the photodegradation of synthetic polymers used as pottants and/or
cover sheets in the LSA Molar cell modull 9 designs. It involves the develop-
ment of a computer simulation of the chemical processes that take place
under weathering cor,.ditions which could, in principle, relate directly to the
performance of these materials and afford some basis for predicting and/or
controlling their useful lifetimes.
The program can be divided into three main parts:
t. The development of a computer program to model the weathering/
photooxidation of an ethylene-vinyl acetate copolymer as a ty!pLeal
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.
r,
"Vts report was prepared as an account of work sponsored by the United States
Covernment, Neither the United States nor the United States Department of 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 its use would not
infringe privately owned rights."
Summary
f	 «
J'PL 955591-- 3
1.	 Computer Simulation of Photooxidation
In the first year of the contract, a computer model which includes
an integration routine originally advanced by Gear* was developed and demon-
strated to simulate, in principle, the chemical changes which may occur in
the photooxidation of hydrocarbons, using as input data a set of elementary
reactions, corresponding rate constants and appropriate starling conditions.
Application of this wilel to the photooxidation of pottant and plastic materials
used in the LSA module designs should provide for the first time a. reliable
predictive capability regarding the useful lifetime of these materials.
During the first quarter of this year the general mechanism of ele-
mentary reactions has been reviewed and improved. The earlier mechan .6m
consisting of 46 reactions has been simplified considerably by reducing the
number of formal termination steps since it became apparent that the major-
termination process goes via the peroxy radicals. Thus, we have eliminated
all the ether radical t/arminations and disproportionations, retaining only the
following:
ROO • + ROO - —> RO4R	 (1)
CROO • + CROO , 	 > Disproportionation (in cage) (2)
RO • + RO • > ROOR (crosslinking) (3)
R • + R- RR	 " (4)
R- + RO + > ROR (5)
In addition,, we have included new reactions of oxygen with acyl radicals
(from Newrish type 1) to form peracids, which then decompose to form
carbon dioxide:
`:	 * C. W. Gear, Comm. ACM, 14, 176 (1971).
JPL 955591 -- 4
RCO • + 02 ---> R0000 • 	 (6)
^	 R0000 • + RH ---> Peracid + R •	 (7)
Peracid h^=> RCOO • + OH • 	 (8)
RCOO • ----> R • + CO2	(9)
We have also eliminated the sensitized decomposition of BOOR by enermr trans-
fer from excited ketonev, and reduced the value of the rate constant for the
sensitized decomposition of ROOH (k= 108) . The photochemical reactions
of small ketones formed by the Norrish type tt process have also been added
for completeness. The result Is that we now have to deal with 36 reactions
(down from 46) and 30 different species (down from 33) .
Having charged the mechanism significantly, the integration parameters
require manipulation to afford appropriate results on a realistic time-scale.
We are now in the process of gaining hands-on experience of varying the inte-
gration parameters by feeding different input data to the computer. No
dramatic differences have been observed to modify any of our earlier conclu-
sions, namely that the major products predicted are ketones, alcohols and
water with both scission by ketone cleavage and some crosslinking by radical
recombination takLig place (see Appendix) .
The computer output shows the predicted concentration profiles of all
the chemical species involved over time but the short time scale of days and
hours implies me -°: work on our model is necessary.
2.	 Photooxidation of Model Alkanes
We have continued to monitor the photooxidation of n-decane and 2, 4
dimethyl pentane (DMP) initiated with peroxide and/or ketone as models for
the photooxidation of polyethylene and ethylene segments of EVA in order to
validate the general mechanism and computer product simulation.
The major products have been established earlier to be isomeric ketones,
alcohols and water. To get clearer insight into the mechanism we have been
«
	 JPL 955591 -- 5
doing finger-print analysis on the GC tm Identify the other products of photo-
; oxidation. This has proven to be an elusive exercise to date, but otw efforts
are continuing. We have also made preparation far LC analysis to attempt
more effective and reproducible product separation. Some unavailable
expected products are currently being synthesized for use as standards.
3.	 EVA Automat id Viscometry
We have irradiated solutions of ELVAX in methylene chloride and
measured the viscosity for different wnounts of irradiation time In automated
sequence. The neat ELVAX in solution shows a linear increase in viscosity
with time of irradiation. This implies that the EVA crosslinks to some degree
on irradiation (Figure 1) .
Samples of additives used in the formulation to stabilize EVA have been
supplied from Springborn Laboratories. We have also measured the viscosity
changes In solutions of ELVAX containing up to 10% NAUGARD P (Uniroyal)
or Cyasorb UV 531 (a substituted benzophenone from American Cyanamid) .
There is a similar linear increase in viscosity of these solutions showing
the absence of any stabilization effect in solution.
The observation of increased viscosity, which strongly suggests that
crosslinking is a major process, implies that there is a mono 111,wly possibility
of the practical degradation of these materials deteriorating by shrinking
and stripping from supports as the crosslin ing takes place rather than the
alternative embrittlement normally due to polymer scission and lower molecu-
lar weight.
We would now like to irradiate films of ELVAX with and without the
additives and then measure the viscosities of the solutions formed by dissolving
the irradiated films to measure the effectiveness of these stabilizers on EVA
in the solid state.
Exposure of both clear and white :EVA film samples to outdoor weathering
conditions is being continued at EcuPlastics Limited. Some weatherometer
e.Voriments are also being + mmed.
w•4
IF ,	 0.78
0.76
9M
r^
u 0.74
t
0.72
0.70 40
A with Norgard
O ELVAX
q with Cyasorb UV 531
6	 12	 1$	 24	 30	 36
Time (hours)
FIGURE 1. Intrinsic viscosity vs. irradiation time
(ELVAX 0.47 g/100 ml CH2Cl^
1
rAppendix
Some Examples of Input-Output Data Used in the Current.
Simulation RoutiYie for lAkane Photooxidution
p.	 i
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'►
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1 I2 U1l + R1i -> R + HUH
2 P2	 ' kO0 + RH -> R + ROOM
3 P1 R + U2 -> ROO
A h4 R + ROUH - > ROO + RH
5 PS RUCH -> RO + ON
4 P6 RO + RH -> RON + R
7 P7 ROO + Rfla -> R04R
4 8 P8 CROO.^ + CRCO -> CRO + CRO	 + 5U2
9 P9 R04,R -J CROO + CROO
ly P10 CROO + CROO -> RON + KETONE	 + S02I
11 P11 CItO -> RO
12 P12 ROO + ROO -> RON + KETONE	 + S02
13 P13 RO -> R + KETONE
I
14 P14 KETONE -> KET
15 P141 SMKETC14E -> KET
i 1 f. 1, 1 5 K ET -> SI.1Ft + RCO
1 17 P 151 514R + RH -> SMRH + R 
i s P 1 52 RCO + RH -> ALDEHYDE + R
19 P 153 Acc -> SIAR + CO
?O P154 P.CO + 02 -> P.CCDO
21 P155 N0000 + RH ^> PERACID	 R
_ 22 P156 PER AC I D -> RCOO + OH
23 P157 Mo r-> R + CO2
7_k	 P16 KET -> ALKENE + SMKETONE
25 P 161 KET + 02 -•> KETONE + S02
26 P162 KET -> KETONE
' 27 P 163 S02 -> 02 --
as P13 EtET + RCCH -7 KETONE + RO	 + OH
-9 p 1 9 CRO + CRO ROOR
;0 P20 CROO -> ROO
31 P21 S02 + ALKENE -> ROOH
32 P22 R + ALKEf.E -> BRANCH
-, 3:: T3 P. + OH -> ROOH
34 T6 RO + RC -> R OOR
3.v- T7 R + R RR
?6 T1O R + OR -> ROR
* ^
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Modelling of polymer photodegradation for solar cell modules

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