at SciVerse ScienceDirect

Polymer Degradation and Stability 98 (2013) 720e726

Contents lists available

Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate/polydegstab

Degradation mechanism of silicone glues under UV irradiation andoptions for designing materials with increased stability

Hartmut R. Fischer a,*, Christopher Semprimoschnig b, Cathal Mooney b, Thomas Rohr b,Ernst R.H. van Eck c, Margriet H.W. Verkuijlen c

a TNO Materials Performance, De Rondom 1, 5612 HE Eindoven, The Netherlandsb ESA-ESTEC, TEC-QT, Keplerlaan 1, 2200 AZ Noordwijk, The NetherlandscRadboud University Nijmegen, Institute for Molecules and Materials, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands

a r t i c l e i n f o

Article history:Received 1 November 2012Received in revised form18 December 2012Accepted 20 December 2012Available online 2 January 2013

Keywords:Silicone rubberUV irradiationDegradation mechanismESRSpace environment

* Corresponding author. Tel.: þ31 888665381.E-mail address: hartmut.fischer@tno.nl (H.R. Fisch

0141-3910/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymdegradstab.2012.12.0

a b s t r a c t

The degradation of silicone glues used, for example, in the assembly of solar modules for use in space, hasbeen investigated and possible mechanisms which lead to colouration and possible embrittlement areanalysed. Both effects are connected to the generation of radicals upon exposure to UV irradiation. Asa follow up-reaction in vacuum, cross-linking and subsequently embrittlement and cracking due toshrinkage can occur. The degradation of the polymers via a radical mechanism can also lead to theformation of small metal clusters containing residues of the Karstedt catalyst and also silicon. Both typesof nanoparticles contribute to colouration of the silicone glues. Possible ways to avoid this is the use ofultra-pure starting substances, a minimum amount of catalyst and a higher cross-link density of thesilicone, minimizing the mobility of dangling chains formed upon UV-exposure and their tendency toform metal clusters.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Transparent silicone glues are extremely versatile and hencetechnologically important materials. They display many desirableproperties: adhesion to many different surfaces and low shrinkagewhen they cure. This is coupled with flexibility, good tensilestrength, high elongation and tear strength and especially resist-ance to weathering, ozone, chemicals, abrasion etc. and with theability to maintain these physical properties over a wide temper-ature range from �100 �C to 300 �C. Silicone glues are generallyeasy to process and therefore usable in many different applicationslike encapsulants, sealants or adhesives, especially in cases wherethe unique properties of these glues such as an extremely low glasstransition temperature, low outgassing, thermal stability andexcellent elastomeric properties are required in combination withoptical transparency. In applications like sealants for blue LED’s oradhesives for bonding a protective glass cover onto solar cells usedin space, the functional components are not only exposed to the UVradiation but also to significant stresses due to the differences inthermal expansion coefficients during eclipse phases [1e4].

er).

All rights reserved.22

Therefore, next to good UV-stability, adhesives used for encapsu-lation should maintain a low Young’s modulus even at lowtemperatures.

UV irradiation can cause rupture of bonds in organic molecules.Photochemical reactions in organic molecules may result in effectssuch as discolouration of the material. In addition, the participatingradical reactions due to homolytic bond breakage may also causecolouration, especially when volatile compounds are formed andthe remaining material contains unsaturated bonds which canresult in increased solar absorbance and/or loss of mechanicalproperties due to chemical changes in the material.

Despite the very simple chemical composition (platinum-cata-lysed crosslinked polydimethylsiloxane (PDMS) architecture usinghydrosilane and vinylsilane functionalities) of the industrial ma-terials for such applications, a colour degradation is noted uponsimulation of space environments [1]. The silicone DC 93-500 fromDow Corning as used in space for more than 30 years, showsa visible colouration coupled with an increase in modulus andbrittleness and a slight shrinkage upon intense UV irradiation [1]. Asimilar colouration is observable for the silicones Elastosil S 690and Elastosil S 695 from Wacker Chemie AG [2e5]. The curingchemistry chosen for optical applications produces no by-products,the materials themselves consist of high purity monomers and only

H.R. Fischer et al. / Polymer Degradation and Stability 98 (2013) 720e726 721

a minimum amount of Karstedt catalyst (a complex formed by re-action of chloroplatinic acid and 1,3 divinyltetramethyl-disiloxane).

The goal of this investigation was to attempt to identify thenature of the colour centres which are formed in silicones duringdegradation upon UV irradiation and to understand themechanismof the degradation. Consequently, a design of more stable siliconesbased on the findings is introduced and the first results discussed.

2. Experimental procedures

The composition of the three different silicone glues used in thisstudy is given in Table 1.

All compounds used are ultraclean silicones with almost novolatile components and a minimized concentration of Pt-catalystto ensure the highest possible stability upon UV irradiation. Espe-cially a minimization of the amount of Pt within the reactionmixture results in substantially increased UV-stability as discussedbelow.

A fundamental difference between the Dow and the Wackersilicones is the presence of phenyl groups in the case of the Wackercompounds to prevent crystallisation as well as to reduce the cross-linking efficiency and to reduce the on-set of photo-degradation[6,7]. Although this influences the absorption spectra, especiallyat wavelengths below 250 nm, long term degradation shows onlya limited colouration and also a minimized embrittlement due tocross-linking. However, in contrast to the Dow DC 93-500 theWacker S 695 still shows the presence of residual SieH bonds even6 days after curing at ambient conditions [8].

UV-exposure testing of quartz glass sandwiched glues (thick-ness about 25 microns) was carried out at the European SpaceResearch and Technology Center (ESTEC) Materials Space Evalua-tion and Radiation Effects Section’s TEC-QTE’s SUV facility, whichconsists of a high vacuum chamber (<10�6mbar). The UV sourceconsists of an array of 4 halogen discharge lamps which produceUV, visible and IR radiation. The lamp unit is positioned outside thevacuum chamber, and the radiation is transmitted to the sampleplate through the fused silica window, which transmits UV radia-tion down to 200 nm. The lamps are powered by stabilised powersupplies, which can be operated at a variable wattage from 270 to600 W. Before and after the exposure, the UV intensity wasmeasured on the sample holder inside the facility. The intensitywas measured with three calibrated UV detectors, in the wave-length ranges UV-A (315e400 nm), UV-B (280e315 nm) and UV-C(200e280 nm), at each sample location. The UV intensities werecompared to the solar spectrum [ASTM E490-00a ‘Solar Constantand Zero Air Mass Solar Spectral Irradiance Tables’] and averaged tocalculate the acceleration factor expressed in solar constants. Thethermally controlled sample holder was located in the centre of thechamber; the samples were mounted with the help of a thin Al foilto ensure a good thermal contact to the sample holder. In this waythe sample temperature was kept at 210 � 5 �C. Measurements ofthe thermo-optical properties (spectral emissivity and absorbance[ECSS-Q-ST-70-09C ‘Space product assurance: Measurements ofthermo-optical properties of thermal control materials’]) wereperformed at the intermediate inspection point (first exposure

Table 1Chemical composition of the transparent, space certified silicone glues used in thisstudy.

Silicone sample mol % SiH mol % vinyl mol % phenyl

Dow DC 93,500 1 0.91 e

Wacker Elastosil S 695 0.045 0.144 4Wacker Elastosil S 690 0.05 0.05 0.45

phase: average 7.8 solar constants, 790 equivalent sun hours, esh)and again at end-of-life (EOL) at an additional 2269 esh.

In addition to the sandwiched samples, strips of the silicones(10 � 2 � 2 mm) were exposed using the ESTEC TEC-QTE’s ESRfacility setup for in-situ exposure of materials under vacuum duringthe recording of ESR spectra as described earlier [5] (see Fig. 1).

These samples were subsequently analysed at the national solidstate nuclear magnetic resonance (NMR) facility at Radboud Uni-versity, The Netherlands to detect changes in the silicone structureand the possible formation of new species upon irradiation. Solid-state NMR experiments were performed on 850 MHz/20 T,600 MHz/14.1 T, 400 MHz/9.4 T and 300 MHz/7.0 T Varian VNMRSspectrometers using 3.2/4.0/7.5 mm HXY MAS probes. The 13Csingle pulse excitation spectrawere measured at a magnetic field of20 T using a 90� pulse with a pulse length of 5.5 ms, a radio fre-quency field strength of 50 kHz and spinning the sample at 4 kHz.1H single pulse excitation spectra were measured at a magneticfield of 20 T/9.4 T using a 90� pulsewith a pulse length of 4 ms, an RFfield strength of 62.5 kHz and a sample spinning speed of 4 kHz.The 29Si single pulse excitation spectra were recorded at 20/14.1/7.0 T, using a 90� pulse with a pulse length of 4.2 ms at an RF fieldstrength of 60 kHz and a sample spinning speed of 4 kHz. The 1H,13C and 29Si chemical shifts were referenced with respect to tet-ramethylsilane (TMS). All NMR data processing was done usingmatNMR [9].

Ultra-microtomed samples derived from the exposed strips ofsilicone were inspected by TEM and fracture surfaces wereinspected using an Orion Scanning Helium Ion Microscope (SHIMor HeIM) from Carl Zeiss Microscopy.

Finally, initial experiments were performed to prepare siliconeglues which could offer enhanced stability while increasing thecrosslink density and therefore decreasing the molecular mobilityof the silicone chain units and possible degradation products fol-lowing recently described procedures [10].

3. Results

Exposure of thin sandwiched silicone glue films to UV in vac-uum leads to a substantial colouration. Fig. 2 shows sandwichedsamples of the silicone glues after UV-exposure. This observation isin agreement with earlier investigations [11,12], which reporta certain susceptibility of silicone encapsulants to UV radiationdegradation. The colouration upon UV irradiation seems to bea general feature, nearly independent from the molecular structureof the silicones. The Wacker material shows a more yellowish-

Fig. 1. Picture of stripes of the silicone glues after in-situ UV-exposure under vacuumusing the ESTEC ESR facility, a) Elastosil S 690, b) Elastosil S 695, c) Dow DC 93-500,scale bar 20 mm.

Fig. 2. Picture of the sandwiched samples of the silicone glues after UV-exposure under vacuum using the ESTEC SUV facility, a) Elastosil S 690, b) Elastosil S 695, c) Dow DC 93-500,dimensions 19 � 19 mm, thickness 25 microns.

H.R. Fischer et al. / Polymer Degradation and Stability 98 (2013) 720e726722

brown colouration and the Dow material shows a more greyishbrown colouration. Additionally, cracking of the Dowmaterial uponirradiation is observable indicating a shrinkage/hardening of thedegraded glue.

Fig. 3 shows the UVeVIS spectra obtained from sandwichedsilicone glue samples as recorded for the beginning of life-time(BOL) and different end of life-times (EOL) with increasing timeof exposure.

Colouration in organic substances is mostly connected with thepresence of conjugated unsaturated bonds which offer a highmobility to an electron cloud and therefore the possibility of aneasy excitation with energies equivalent to the optical spectrum.13CeSSeNMR (see Fig. 4) did not show any evidence within thedetection limit of ca. 2% of unsaturated or conjugated speciesderived from ethylene-bridges present as a consequence of thevinyl-hydrosilane coupling which may be a possible option con-cerning the nature of chromophores.

All groups seem to be mobile in the pristine samples, sincenarrow lines and typical J-coupling patterns for CH3 groups areobservable in the NMR spectra. After irradiation, a certain increaseof a fraction of less mobile CH3 groups appear, as shown by thebroad line underneath the methyl quartet. This could well be anindication of radical initiated cross-linking reactions. This matchesvery well with earlier investigations by Hill, Preston andWhittaker,which studied structural changes of poly(dimethyl siloxane) duringgamma radiolysis under vacuum at 303 K using 29Si and 13C NMR[13]; here, mainly chain scission and subsequent cross-linking (H-and Y-type) has been observed; the cross-linking dominates overchain scission. Fig. 5 shows the proton NMR spectra before and afterirradiation.

The emergence of spinning side bands after irradiation could bean indication of reduced mobility. The fact that at a lower magnetic

Fig. 3. UVeVIS spectra of the silicone glues during UV-exposure under vacuum using the ESUVeVIS absorption in the range 270e450 nm (S 690), 270e400 nm (S 695), 200e330 nm

field the spinning sideband intensities do not change implies thatthey are purely dipolar in origin where a reduction in mobilitywould prevent the averaging of the dipolar coupling. If the origin ofthe spinning side bands was due to the increase in paramagneticcontent of the sample the sideband intensities would drop ata lower field.

The formation of Y-crosslinks and of SieCH2eSi bonds is espe-cially favoured; a formation of SieSi-bonds can be found in H-crosslinks (see also [14,15]: Si-NMR: �21.9 ppm PDMS, 7.25 ppmchain ends, �21.4 ppm cyclic compounds, hydrosilane �37.4 ppm,si-eth-Si �23 ppm, SieSi �55 ppm, C NMR 1.8 ppm chain ends,0.6 ppm PDMS). Other studies indicated the formation of methyl-ene bridged siloxane and an increase in cross-linking density uponirradiation in vacuum [11]. 29SieSSeNMR spectra of S 690 showed(Fig. 6) resonances which can be assigned to eCH3SiO2e with twoeCH3SiO2e neighbouring units at �22.5 ppm and eCH3SiO2ewithePhSiO2e andeCH3SiO2e as neighbours at�21.3 ppm. The smallerpeaks are assigned to ePhSiO2e at �47.9 ppm, ePhCH3SiO2e at�35.4 ppm and chain ends at 7.6 ppm [16]. No distinct concen-trations of either hydrosilane groups or ethylene bridged siliconesas well as of SieSi bonded species are observed. Also, possibleformed Si-(nano) particles could not be detected; here separatespectra acquired for Si-nanoparticles with an average size of 30 nmand of 100 nm are also shown in Fig. 6. The absence of the signals ofnanoparticles in the spectrum of the irradiated sample can possiblybe connected and explained by the very low concentration ofpossibly formed particles and the extremely long relaxation timesT1, which makes detection difficult. However, again a broadening oflines due to cross-linking can be detected.

Ultra-microtomed samples inspected by TEM and fracture sur-faces inspected using a Scanning Helium Ion Microscope (SHIM orHeIM) showed the presence of small clusters with higher electron

TEC SUV facility, a) Elastosil S 690, b) Dow DC 93-500. The samples show an increase in(DC 93e500). BOL ¼ beginning of life-time, EOT ¼ end of life-time [25].

Fig. 4. 13C solid state NMR spectra of the sample DC 93-500 before (bottom) and afterirradiation (top). For comparison a spectrum of a silicone rubber sample with siliconenanoparticles is displayed in the middle.

Fig. 6. 29Si solid state NMR spectra of 30/100 nm Si-nanoparticles (measured at 14.1 T),S 690 with nanoparticles added (measured at 20 T) and S 690 after irradiation(measured at 7.0 T), showing no evidence of substantial chemical changes in thesample upon irradiation.

H.R. Fischer et al. / Polymer Degradation and Stability 98 (2013) 720e726 723

density respectively with a higher conductivity for Elastosil S 690(see Fig. 7).

The higher density/conductivity of the particles points to Si, SiePt or Pt metal clusters, however, no enhanced Pt signal intensitiescould be detected while running selected area EDX in the clusters.Also, no distinct particles were recognised in the samples preparedfrom irradiated DC 93-500.

In general, there is little knowledge on the detailed photo-chemistry responsible for the decrease in transmittance of curedsilicones and polysiloxanes after UV-exposure. The kinetics of theformation of chromophoric species or chromophoric impurities hasbeen studied in detail without a determination of the nature of thechromophores themselves [3]. The high energy ionising chemistryof polydimethylsiloxane (PDMS) has been shown to be predomi-nantly controlled by free radical mechanisms [15]. A study dedi-cated to the vacuum UV photolysis of PDMS has indicated thebreak-down of the SieCH3 bond as the most probable reactionoccurring and resulting in the formation of methane and hydrogenas volatile split-off compounds [17]. An irradiation of a methyl

Fig. 5. 1H NMR spectra of DC 93-500 at 850 MHz, before irradiation (bottom) and afterirradiation (middle). The irradiated sample was also measured at 400 MHz to ascertainthe origin of the spinning sideband manifold. For these NMR measurements, a spin-ning speed of 4 kHz was applied.

phenyl silicone yielded less generation of hydrogen due to anaddition of hydrogen to the phenyl groups in a subsequent reaction.Both materials were vacuum distilled and hence also free of im-purities e.g. catalyst etc. Other studies indicated an increase inhardness upon UV irradiation, hinting to a cross-linking via radicalmechanisms [18].

The exposure of silicones to UV in vacuum is indeed coupledwith the generation and existence of several types of unpairedelectrons (radicals) upon irradiation detectable by ESR spectro-scopy [5,19]. Also in our experiments, several different and dis-tinctive radical species developed during exposure (see Fig. 8).Additionally, line broadening occurs, hinting to the formation ofamorphous carbon and/or to the presence of SiO2.

The location of the most prominent signal is coupled with theformation of eSieCH2$ radicals next to signals showing thedevelopment of eSi$ radicals and of colour centres formed in thequartz glass during and after irradiation. The intensity of this signalwas followed over time using a t-scan (time evolution) exper-imental setup, and is plotted in Fig. 9.

The resulting increase in intensity follows a first-order kineticlaw, following Equation (1):

I ¼ aþ b�1� e�cx

�(1)

as could be expected from such a photolytic initiated degradationreaction where radicals are formed directly by degradation ofa single component as also previously done for such an ESR analysisof this degradation mechanism [5] as well as to describe thechanges in transmittance [1e3] and the increase in brittleness of DC93-500 upon UV irradiation [3]. There, the ultimate tensilestrength, elongation at failure, and transmittance was found todecrease with increasing exposure, and the rate of degradationwasfound to be independent of intensity. The mechanical propertiesand optical properties decreased, approaching asymptotic valuesnear the 2000 equivalent sun hours (ESH) exposure level.

The degradation mechanism of the silicones is consequentlyconnected with a radical mechanism following a homolytic split-ting of covalent bonds initiated via the irradiation with a high en-ergy beam. The vacuum present prevents fast follow-up reactionswhile removing methyl and hydrogen radicals. However since the

Fig. 7. a) HIM and b) TEM images of fractured/microtomed samples of UV-exposed Elastosil S 690. Clearly the presence of small particles/clusters of increased electron density isobservable hinting to the presence of small metal clusters.

H.R. Fischer et al. / Polymer Degradation and Stability 98 (2013) 720e726724

reactions take place in a solid, the diffusion of other reactive radi-cals (eSieCH2$, eSi$) is probably limited and forces follow-up re-actions possibly connected with a formation of extendedconjugated systems as assumed by Zimmermann [3]. A similarprocess may cause the formation of metal nanoclusters as well;here the mobility of the rubbery system may enable the possibilityto form large enough clusters. Already an agglomeration of fouratoms forms a quantum dot, which acts as a colour centre.

The colouration by formation of metal nanoparticles (clusters) isaccording to the analysis of silicone glues in this study probably themost likely mechanism. It is known that the interaction of thevinylsilanes and especially the hydrosilanes with the used Pt-containing Karstedt catalyst can lead to the formation of nano-Ptclusters (size 2e4 nm) which are coloured. In case of excess ofhydrosilanes is the formation of SiePt clusters favoured due to thefirst step of the reaction of the catalyst with the hydrosilane underformation of a direct SiePt bond connected with an increase inabsorption at l¼ 300e500 nm. Silicon nanoparticles absorb light inthis spectral range. The difference between the Dow and theWacker glues can be seen in the composition, the Dow glue con-tains a slightly smaller than stoichiometric amount of vinyl-groups.

-4000000

-3000000

-2000000

-1000000

0

1000000

2000000

3000000

3200 3250 3300 3350 3400 3450 3500

Magnetic field [G]

ES

R s

ign

al in

te

ns

ity

[a

.u.]

Fig. 8. In-situ ESR spectrum of UV-exposed Elastosil S 690. The red circle indicates theposition of the signal generated from eSieCH2$ radicals. (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version ofthis article.)

Since the hydrosilanes are involved also in side-reactions, this leadsmost likely to an equivalence of the two reactive groups andtherefore to only a minor formation of rather pure Pt-nanoclusterswhich are brown-greyish coloured. Such Pt clusters were formede.g. just by degradation of the Karstedt catalyst and display thedescribed colour [20]. The platinum end product of hydrosilylationreactions is a function of the stoichiometry of the reactants. Inter-estingly, no SAXS scattering has been observed while analysingsolutions containing Karstedt’s catalyst during hydrosilylation [21].Hydrosilylation in the presence of excess olefin yields a platinumproduct in an olefin rich environment originated from PteC bondsand in the presence of excess hydrosilane yields platinum ina platinum and silicon rich environment originating from multi-nuclear Pt clusters with silyl ligands [20,22,23]. Transmissionelectron microscopy (TEM) of evaporated solutions of hydro-silylation reaction mixtures showed the presence of colloidalcrystallites in both cases (excess olefine 40�A, excess hydrosilanee25 �A, see Fig. 10). The origin of the difference in size is not known.The colloidal Pt formed under the condition of excess hydrosilane isbrown, while that formed under excess olefin condition is darkbrown [22,24].

Fig. 9. In-situ ESR t-scan (time evolution) of the signal generated from eSieCH2$

radicals upon irradiation of S 690, axis time in s and intensity of the signal.

Fig. 10. TEM pictures of Pt-nanoparticles obtained by degradation of Karstedt catalyst during exposure to air. Scale bars 20 nm.

-1000000

-800000

-600000

-400000

-200000

0

200000

400000

600000

3200 3250 3300 3350 3400 3450 3500

Magnetic field [G]

ES

R sig

nal in

ten

sity [a.u

.]

silicone 5-1empty vial

Fig. 12. In-situ ESR spectrum of UV-exposed model silicon glue using a ratio of 5:1 ofthe cyclic monomers DH

4 /DV4 . Besides the signals originated from the exposure of the

glass tube also a signal originated from Si-radicals could be observed.

H.R. Fischer et al. / Polymer Degradation and Stability 98 (2013) 720e726 725

The Wacker silicones contain no excess of hydrosilanes, and incontrast to the situation for DC 93 500, wheremost likely Pt clustersare formed during degradation, in these cases a formation of SiePtand CePt clusters which are yellowish-brown coloured occurs.Possibly and most likely, also present phenyl groups are in closeproximity to the metal clusters, this would explain the difference incolour of the degraded Wacker silicones compared to the DOW DC93-500. It seems that this colouration is slightly more intense thanthe colouration originated by the Pt clusters. On the other sidethere is no cracking of the degraded glue layer observable as in thecase of irradiated Dow 93-500; again, back-biting and splitting offof volatile cyclic oligomers is prevented by the presence ofphenylsilanes.

A minimisation of the amount of Pt-catalyst to a level justenough to catalyse the reaction to ensure curing in a reasonabletime (about one day) coupled with a equimolarity of both compo-nents or a slight excess of hydrosilane and an increase in stiffness ofthe silicone glue and such minimizing the mobility of the chainsduring the irradiation and their possibilities to form Pt-(Si) clusters

Fig. 11. Photograph of a sandwiched sample of a highly crosslinked model silicone glueusing a ratio of 2:1 of the cyclic monomers DH

4 /DV4 after about 300 esh UV-exposure.

Dimensions 19 � 19 mm, No colouration was observable.

should consequently result in a much better stability against UVirradiation. One of the possible ways to achieve this is the use ofcyclic monomers which minimizes the amount of carbon withinthe siloxane network and gives thewidest opportunities to tune thestiffness of the thermosetting network. Initial experiments usingthe platinum-catalysed hydrosilylation reaction between 1,3,5,7-tetramethylcyclotetrasiloxane.

(DH4 ) and 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasilox-

ane (DV4) confirmed that. A compound obtained while using a ratio

of 2:1 of the cyclic monomers DH4 /D

V4 yielded a colourless trans-

parent glassy solid, which, although showing the development ofradicals within the ESR setup, shows no signs of colouration afterUV irradiation between quartz glass sandwiched samples for about3000 esh in vacuum (Fig. 11).

As expected, the resulting resin is by far less sensitive to expo-sure to UV-light.

A difference is the stability of a compound obtained using a ratioof 5:1 of the cyclic hydrosilane/vinylsilane tetramers. Here, justa dominance of Si-radicals next to colour centres formed in thequartz glass during and after irradiation is observable in the ESRspectra (see Fig. 12).

Consequently, this compound also showed colouration afterUV-exposure.

4. Conclusions

The degradation of silicone glues used e.g. for the assembly ofsolar modules for use in space as manifested by colouration and

H.R. Fischer et al. / Polymer Degradation and Stability 98 (2013) 720e726726

possible embrittlement is connectedwith the generation of radicalsupon exposure to UV irradiation. As a follow up-reaction, cross-linking and subsequently embrittlement and cracking due toshrinkage can occur. The degradation of the polymers via a radicalmechanism can also lead to the formation of small metal clusterscontaining residues of the Karstedt catalyst and also silicon. Bothtypes of nanoparticles contribute to a colouration of the siliconglues. Possible ways to avoid this is a use of ultra-pure startingsubstances, a minimum amount of catalyst and a denser cross-linkdensity of the silicon minimising the mobility of dangling chainsformed upon UV-exposure and their tendency to form metalclusters.

Acknowledgement

We thank Hans v.d. Veer, TNO, for the HIM pictures, HelmutGnägi, Diatom, the “wizard” for the ultramicrotome cutting for thepreparation of the TEM samples and Dr. Markus Drechsler, Uni-versity Bayreuth and Bayreuther Institut für Makromolekülfor-schung for the TEM pictures. Support of NWO for the “Solid StateNMR facility for Advanced Materials Science” is gratefullyacknowledged.

References

[1] Dever JA, Banks BA, Li Y. Effects of vacuum UV radiation on DC 93-500 silicone.J Spacecrafts Rockets 2006;43:386.

[2] Zimmermann CG. Materials challenges in photovoltaic energy generation inspace. MRS Bull 2010;35:48e54.

[3] Zimmermann CG, Pueyo D, Semprimoschnig C, Taylor S. End-of-life perfor-mance evaluation of a new coverglass adhesive. European Space Agency; 2008(Special Publication) ESA SP661.

[4] Semprimoschnig COA, Heltzel S, Polsak A, Van Eesbeek M. Space environ-mental testing of thermal control foils at extreme temperatures. In: Pro-ceedings of the 9th ISMSE; 2003. p. 161e7.

[5] Rasmussen K, Grampp G, Van Eesbeek M, Rohr T. Thermal and UV degradationof polyimides and silicones studied in-situ with ESR spectroscopy. In: Pro-ceedings of the 11th ISMSE; 2009.

[6] Delides CG. The protective effect of phenyl group on the crosslinking ofirradiated dimethyldiphenylsiloxane. Radiat Phys Chem 1980;16(5):345e52.

[7] Koike M, Danno A. Radiation effects on dimethyl-diphenyl siloxane copoly-mer. I. Protective effect of phenyl radical on the cross-linking. J Phys Soc Jpn1960;15:1501.

[8] Internal report ESTEC on composition and reactivity of silicone glues used inspace applications.

[9] Beek JD. matNMR: a flexible toolbox for processing, analysing and visualizingmagnetic resonance data in Matlab�. J Magn Reson 2007;187:19e26.

[10] Zheng P, McCarthy TJ. Rediscovering silicones: molecularly smooth, low sur-face energy, unfilled, UV/Vis-transparent, extremely cross-linked, thermallystable, hard, elastic PDMS. Langmuir 2010;26:18585e90.

[11] Delman AD, Landy M, Simms BB. Photodecomposition of polymethylsiloxane.J Polym Sci A 1969;7:3375e86.

[12] Chien A, Maxwell R, Chambers D, Balazs B, LeMay J. Characterization of ra-diation induced ageing in silica reinforced polsiloxane composites. RadiatPhys Chem 2000;59:493e500.

[13] Hill DT, Preston CML, Whittaker AK. NMR study of the gamma radiolysis ofpoly(dimethyl siloxane) under vacuum at 303K. Polymer 2002;42:1051e9.

[14] J Ma. Chem Phys 1994;101:2734e9.[15] Miller AA. Radiation chemistry of polydimethylsiloxane.1 I. Crosslinking and

gas yields. J Am Chem Soc 1960;82:3519.[16] Engelhardt G, Mägi M, Lippmaa E. 29Si-NMR-spektroskopische untersuchun-

gen an methyl- und methyl-phenyl-siloxanen und ihre anwendung in dersilikonanalytik. J Organomet Chem 1973;54:115e22.

[17] Siegel S, Stewart T. Vacuum-ultraviolet photolysis of polydimethylsiloxane.Gas yields and energy transfer. J Phys Chem 1969;73:823.

[18] Virlogeux F, Bianchini D, Delor-Jestin F, Baba M, Lacoste J. Evaluation of cross-linking after accelerated photo-ageing of silicone rubber. Polym Intern 2004;53:163e8.

[19] Menhofer H, Heusinger M. Radical formation in polydimethylsiloxanes andpolydimethylphenylsiloxanes studies by the ESR spintrap technique. RadiatPhys Chem 1987;29:243e51.

[20] Stein J, Lewis LN, Gao Y, Scott RA. In situ determination of the active catalyst inhydrosilylation reactions using highly reactive Pt(0) catalyst precursors. J AmChem Soc 1999;121:3693e703.

[21] Lewis LN, Stein J, Smith KA, Messmer RP, LeGrand DG, Scott RA. Recentmechanism studies on hydrosilyation. In: Marciniec B, Chojnowski J, editors.Progress in organosilicon chemistry. Amsterdam: Gordon and Breach; 1995.p. 263.

[22] Lewis LN, Stein J, Gao Y, Colborn RE, Hutchins G. Platinum catalysts used in thesilicones industry, their synthesis and activity in hydrosilylation. PlatinumMetals Rev 1997;41:66e75.

[23] Li C, Li D, Zhao ZS, Duan XM, Hou W. Platinum nanoparticles from hydro-silylation reaction: carbosilane dendrimer as capping agent. Colloids Surf A:Physicochem Eng Aspects 2010;366:45e9.

[24] Devi GS, Rao VO. Room temperature synthesis of colloidal platinum nano-particles. Bull Mater Soc 2000;23:467e70.

[25] Semprimoschnig COA, Pueyo D, Williamson JR, Mooney C, Zimmermann C,Eesbeek Mv, et al. Elastosil S 690 e a space qualified European silicone ma-terial. In: 11th ISMSE, paper S7, France; 2009.