DEVELOPMENT OF A NEW SILICONE ADHESIVE FOR SPACE USE: MAPSIL QS O. Guillaumon 1

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DEVELPMENT F A NEW SILICNE ADHESIVE FR SPACE USE: MAPSIL QS 1123. Guillaumon 1 S. Remaury 2, P. Nabarra 2, P. Guigue-Joguet 2, H.

com (2)Centre National d Etudes Spatiales (CNES) 18 av Edouard Belin 3141 Toulouse Cedex 9 (France) Tel: 33()561273133 Fax: 33()561273446 stephanie.remaury@cnes.

The technologies that have been developed for space industry and for defense being quite close, it is usual to find the same products in both fields.

1 DEVELPMENT F A NEW SILICNE ADHESIVE FR SPACE USE: MAPSIL QS Guillaumon 1 S. Remaury 2, P. Nabarra 2, P. Guigue-Joguet 2, H. Combes 2 (1)MAP (Space and High tech coatings) ZI Rue Clément Ader-91 PAMIERS (France) Tél : 33() Fax : 33() o.guillaumon@map-coatings.com (2)Centre National d Etudes Spatiales (CNES) 18 av Edouard Belin 3141 Toulouse Cedex 9 (France) Tel: 33() Fax: 33() stephanie.remaury@cnes.fr 1 BINTRDUCTIN Historically, space industry has qualified, for its proper use, many USA products, which have a well-known anteriority. The technologies that have been developed for space industry and for defense being quite close, it is usual to find the same products in both fields. For strategic reasons, the US government has decided to restrict or even to prohibit the distribution of some parts of its own products in some geographic areas (I.TA.R International Trade Arm Regulation). Therefore, the use of such products, for non-us space projects, should be of high risk in terms of availability. Moreover, the chemical industry is about to undergo great changes, specially in Europe. Indeed, in order to meet the requirements of various directives, the European Parliament has set that companies have to control VC (Volatile rganic Compounds) emissions and reduce the use of dangerous materials, while following the REACH program (Registration Evaluation Authorization of CHemicals). This last regulation also concerns space coatings. As a leader in the formulation of high tech coatings, MAP Company has created a research unit dedicated to the silicone polymers synthesis for space industry. Thanks to partnerships with CNES (Materials & Coatings Laboratory), ESA and Universities, MAP has managed to provide manufacturers with innovating and competitive solutions, while contributing to strategic European technological independence. The first development is a new low outgassing silicone elastomer for the potting, varnishing and bonding of space (Fig. 1.) and ultra vacuum uses. This coating is called MAPSIL QS This product may find applications in electrical, thermal insulation or vibration shock dampening, bonding of different substrates (i.e. cover-glass, SR, glassglass ), atomic oxygen protection, optical transparency and resistance to space environment (UV, particles, γ radiations), PCB protection against moisture, encapsulation of electronic components Fig. 1. Pictures of encapsulating, varnishing, and bonding. With its very low outgassing rate (RML<.1% et CVCM<.1%), this coating is quite suitable for optical applications. MAPSIL coating may be applied in : Thin layers (< 5µm), Medium layers (<1 mm), Thick layers (>1 mm). Also, the chemical composition allows to obtain various ranges of mechanical and physical properties that could meet the most varied technical requirements. In this paper, the product development process, the qualification status and the characteristics of MAPSIL QS 1123 are exposed. 2 1BTHE MAPSIL QS1123 TECHNLGY MAPSIL QS1123 is a two-component silicone product that is obtained by hydrosilylation.

2 In order to cure, two other polymers must be added to the product: A polysiloxane with vinyl groups (part A = base). A curing agent (= part B) with at least two - H links. Platinum supported catalysts, in particular the ones used for the hydrogenation of olefins (i.e. Pt/C), are known not to be not very active to catalyze the hydrosilylation. However, in the 195 s, Speier found out that chloroplatinic acid in solution (H2PtCl6 in isopropanol) was much more efficient. More recently, several other catalysts, typically based on species of Pt have shown a great capacity to catalyze the hydrosilylation reaction. In the absence of inhibitor, the hydrosilylation reaction, which can be done without any solvent, is very rapid and under very soft conditions (less than one minute at room temperature with 1 ppm of platinum). Intensity 3,5 3 2,5 2 1,5 1, Wavelength (nm) DC 935 MAPSIL QS 1123 S69 Fig. 2. UV spectra of DC 935, MAPSIL QS 1123 and ELEASTSIL RTV- S H [Pt] TA All the properties of the silicone resin are given after 4 days curing at 8 C. 3 2BPRE-CHARACTERIZATIN F CMMERCIAL SILICNE RESINES AND MAP FRMULATINS The pre-characterization is intended to validate the suitability of MAP silicones for Space use (outgassing) and compare their main characteristics to those of commercial silicones. 4 3BMLECULAR CHARACTERIZATINS F MAPSIL QS 1123 Various spectroscopic methods were used during the development of molecules used in the formulation of MAPSIL QS 1123 to control the quality and the quantity aspects. The RMN 1H, 29 and UV spectroscopy (Fig. 2.) directed qualitative research while gel permeation chromatography (GPC) gave various quantitative information (Fig. 3.). Fig. 3. GPC spectra of the MAPSIL QS 1123 base. 5 4BINITIAL PRPERTIES F MAPSIL QS1123 SILICNE RESIN 5.1 7BPhysico-chemical properties The viscosity was measured using a rheometer with a plane cone at 25 C [1]. This method shows the rheological behaviour of a formulation and whether it is only made of polymers (generally Newtonian behaviour) or loaded polymers (thixotropic or rheoliquid behaviour). The «pot-life» measurement allows to classify silicone resins according to their curing speed. This classification is through the measurement of reaction time required to double viscosity. This measurement was performed with the same equipment as mentioned previously, always at 25 C [1]. The main physico-chemical properties are listed in Table 1. η (viscosity part A) 7 ± 5 25 C η (viscosity part B) 1 ± 5 25 C Pot-life About 9 25 C Table 1. Physico-chemical properties of MAPSIL QS 1123.

3 5.2 8BMechanical properties Shore A hardness was measured according to the NF EN IS 868 standard. The magnitudes of tensile strength and elongation at break were achieved through the tensile test according to the standard NF IS 37. Tensile strength is used to quantify the material cohesion and elongation characterizes the material elasticity. The CNES DCT/AQ/LE department will measure the shear constraint on aluminum test samples (IS 4587 standard) glued by Astrium SAS Toulouse [1]. The main mechanical properties are summarized in Table 2. Hardness Shore A 5 ± 1 σ (Tensile strength at break) 9 ± 1 MPa ε (Elongation at break) 15 ± 2 % Table 2. Mechanical properties of MAPSIL QS Butgassing properties Intespace carried out the outgassing test according to the ESA ECSS-Q-7-2A standard. It was carried out on plots of resin of 3 mm in diameter and 2 mm thick. The results are given in Table 3. TML.6% RML.5% CVCM.% Table 3. utgassing properties of MAPSIL QS The test complies with the ECSS standard, which specify TML and RML values < 1% and CVCM value <.1%. The CVCM value not being measurable, this resin meets the outgassing constraint requirements for optical applications BElectrical properties These properties were carried out on disks of 7 mm in diameter and 5 µm thick. The dielectric strength was measured according to the ASTM D149-97a standard. The permittivity and the loss factors were measured according to the ASTM D15-98 standard. The HP16451B dielectric test fixture was coupled to an HP4284 LCR meter in order to perform the impedance measurements [1]. The volume resistivity was measured according to the ASTM-D standard, the measuring cell used is a plane-plane type with a guarding electrode from Keithley referenced Keithley 89 [1]. These measurements were performed by the Laplace Laboratory from Paul Sabatier University in Toulouse. The surface electrical resistance was measured according to the ASTM D standard by DCT/ TV/TH on 15 µm thick resin films applied on glass [1]. The values are summarized in Table 4. strength ± 5.7 kv/mm* permittivity 1 Hz permittivity 1 khz dissipation factor 1 Hz dissipation factor 1 khz Volume resistivity ohm.cm Surface resistivity 14 Gohm/square Confidence interval 9%: kv/mm Table 4. Electrical properties of MAPSIL QS BThermal properties The vacuum thermal conductivity was measured by the CNES DCT/TV/TH department on a plot of 5x5 mm and 15 mm thick. For the measuring method and the results, we refer to [2]. The coefficient of thermal expansion and glass transition temperature of the resin will be measured by ESA. We will issue these results in a forthcoming edition of this report Bptical properties The CNES DCT/AQ/LE department measured the transmission of a 15 µm thick film of resin applied on a glass sample plate between 2 and 25 nm using a spectrophotometer Varian ref. CARY 5 dual beam (scan speed : 12 nm/min, acquisition step : 1 nm). This measurement showed a resin transmission above 98% between 35 and 15 nm. The transmission spectrum is given in Fig. 4. Transmission (%) Wavelength (nm) Fig. 4. Transmission spectrum of MAPSIL QS 1123.

4 6 5BAGEING TESTS IN SPACE ENVIRNMENT F THE MAPSIL QS1123 SILICNE RESIN Bqualification plan The qualification plan was based on space applications for this resin, namely, potting, varnishing and bonding, also taking into account the characterization and testing means available within CNES premises. Table 5 summarizes the qualification tests performed and the properties checked after the tests. Tests/Properties Mechanical Electrical ptical Thermal cycling X under vacuum Thermal cycling under X X X atmospheric pressure Damp heat X X X Gamma radiation X X UV + particles radiation Table 5. Tests performed on MAPSIL QS1123. Thermooptical The following paragraphs describe in detail the test conditions BThermal cycling under vacuum This test aims to test the resistance of MAPSIL QS1123 silicone resin to temperature as varnish. The resin was applied to aluminum substrate and quartz polyimide substrate (representative substrate of electronic circuit boards, PCBs) in high and low thickness. The adhesion was the reference test BConditions of application The Alu-D primer (on aluminium substrate) and the Kapt-A primer (on quartz polyimide) were applied on the substrates cleaned with a covering time of 1 hour. The spraying procedure was the PSX primer s one (refer to MAP datasheet). The MAPSIL QS1123 silicone resin was applied in high thickness (~ 2 µm) and low thickness (~ 5 µm) according to MAP datasheets respectively of Mapsil- 213B (brush application) and Mapatox-41B (spray gun application). The conditions of application of primers and varnishes were as follows: T = 21 C and humidity = 43% RH. The drying cycles of these varnishes were 3 or 4 days at 8 C. X BTest conditions The thermal cycling tests were carried out at the laboratory of Thermal Environment (CNES DCT/TV/TH). A CNES technical note will report these tests. Test conditions: pressure 1-6 Torr Temperature: -133 C/+116 C Number of cycles: 2 Duration of a cycle : 9 minutes (15 mn cold level, 3 mn increase of 1 C/mn, 15 mn hot level and 3 mn decrease same speed) the drying cycle, and before the thermal cycling, all the samples were stored at 2 C ± 2 C with a relative humidity of 5% ± 5% for several days. The adhesion was tested according to the IS249 standard, squaring every 1 mm for thicknesses less than 1 µm and a every 2 mm for greater thicknesses B test results Tables 6 and 7 give the results. Varnish System Alu-D + MAPSIL QS1123 Substrate Alu AU4G Thickness (µm) Alu-D + MAPSIL QS1123 Alu AU4G 221 Kapt-A + MAPSIL QS1123 Quartz polyimide 48 Kapt-A + MAPSIL QS1123 Quartz polyimide 25 Table 6. MAPSIL QS1123 samples description. Varnish System Adhesion Adhesion Before* * Alu-D + MAPSIL QS1123 Class /5 Class /5 Alu-D + MAPSIL QS1123 Class /5 Class /5 Kapt-A + MAPSIL QS1123 Class -1/5 Class 1/5 Kapt-A + MAPSIL QS1123 Class -1/5 Class 1/5 *(3M 92 Kapton tape) Table 7. Results after thermal cycling under vacuum. The results showed a good adhesion of the Alu-D primer with MAPSIL QS1123 on aluminum substrate and the Kapt-A primer on quartz polyimide substrate between -133 C and +116 C BThermal cycling under atmospheric pressure This test aims to evaluate the resistance to temperature of MAPSIL QS1123, when used for bonding and potting. The optical, electrical and mechanical properties were the test criteria. For this test, mechanical properties could not be measured, but will be tested in a forthcoming test campaign. The test samples and methods for measuring the properties were identical to those described in 5. 48

5 BTest conditions The thermal cycling tests were carried out at the CNES laboratory of Thermal Environment (DCT/TV/TH) [3]. Test conditions: Pressure: atmospheric pressure under nitrogen Temperature: -17 C/+2 C Max number of cycles: 456 Duration of a cycle : 5 minutes (1 mn cold level, 15 mn increase of 25 C/mn, 1 mn hot level and 15 mn decrease same speed) Before the thermal cycling, all the samples were stored at 2 C ± 2 C with a relative humidity of 5% ± 2% for several days BResults The results are described in Table cycles 24 cycles 456 cycles strength (1) (2) (3) Hz khz dissipation factor Hz dissipation factor khz Volume resistivity Surface resistivity to 12 Transmission - - > 92% (1) Confidence interval 9%: kv/mm (2) Confidence interval 9%: kv/mm (3) Confidence interval 9%: kv/mm Table 8. Results after thermal cycling under atmospheric pressure. The thermal cycling at atmospheric pressure between - 17 C and +2 C showed no significant change of the properties of the MAPSIL QS1123 resin BDamp heat This test aims to evaluate the impact of the humidity on the MAPSIL QS1123 silicone resin when used for bonding and potting. The optical, electrical and mechanical properties were the test criteria. For this test, the mechanical properties could not be measured, but will be tested in a forthcoming test campaign. The test samples and methods for measuring the properties were identical to those described in BTest conditions The damp heat test was carried out by the CNES laboratory of Expertise (DCT/AQ/LE). Test conditions: Pressure: atmospheric Temperature: 7 C Number of days: 7 days % relative humidity: 95% Before damp heat testing, the samples were stored at 2 C ± 2 C with a relative humidity of 5% ± 2% for several days BResults The results are described in Table 9. strength 1 Hz 1 khz dissipation 1 Hz dissipation 1 khz Initial value kv/mm damp heat 37.4* Volume resistivity ohm.cm Surface resistivity 14 Gohm/square 11 Transmission > 98% > 98% * Confidence interval 9%: kv/mm Table 9. Results after damp heat test The damp heat test did not show any significant change in the MAPSIL QS1123 resin properties BGamma ray irradiation This test aims to assess the impact of gamma radiation on MAPSIL QS1123 when used for potting. The electrical properties were the test criteria. The test samples and methods for measuring the properties were identical to those described in BTest conditions The test of gamma-ray irradiation was carried out by the CNES DCT/AQ /EC department. Test conditions: Pressure: atmospheric Temperature: room temperature Speed (krad () / h): 4.2 equivalent dose: 1 and 3 krad Before this test, the test samples were stored at 2 C ± 2 C with a relative humidity of 5% ± 2% for several days.

6 Results B The results are given in Table 1. strength Initial value kv/mm irradiation 1 krad irradiation 3 krad Hz khz dissipation 1 Hz dissipation 1 khz Volume resistivity ohm.cm Table 1. Results after gamma rays irradiation. The gamma-ray irradiation did not show any significant change in the MAPSIL QS1123 resin dielectric properties BUV and particules radiation This test aims to evaluate the impact of UV radiation and particles on the MAPSIL QS1123 silicone resin when used for bonding. The tested samples were sandwiches of two type CM cover glasses (from QI ptics) bonded with MAPSIL QS1123. Galileo Avionica made this bonding according to their current (confidential) industrial process used for the Dow Corning DC935 resin. For comparison, a test sample was also made with the latter resin BTest conditions This test was carried out using Sémiramis simulation facility of the Space Environment Department (DESP) from NERA - Toulouse. It simulated a period equivalent to 8 years on N/S faces of a spacecraft in geostationary orbit. A total of 8896 esh was applied with an acceleration factor of about 7-8 and an uniformity of the irradiated area lower than ±1%. The dose profile of space for one year was simulated with one electron energy (in the sample thickness) and two proton energies (on the sample): electrons/cm 2, energy of 4 kev, flux e - /cm 2 /s protons/cm 2, energy 24 kev, flux p + /cm 2 /s protons/cm 2, energy 45 kev, flux p + /cm 2 /s The transmission measurements were performed with a Perkin Elmer Lambda 19 spectrophotometer before and after 8 years GE BResults Effect of the adhesives transmission on the solar cells performances The performance degradation of the solar cell is a short-circuit current degradation, generally proportional to the cell illumination. Modern cells are triple-junction cells. The degradation of the current cell is the junction which limits the current. In most cases, this is the "top" sensitive junction in the nm band. The current density of the junction is calculated by the formula:aerreur! gnet non défini.a, j = with: S( λ). Rs( λ). T ( λ). dλ S(λ) : solar spectrum (1) Rs(λ) : spectral answer of the cell (before irradiation, at 25 C) T(λ) : transmission Fig. 5. shows the transmissions of adhesive sandwiches tested and on the same graph, the typical spectral answer of the "top" junction-type of a cell AzurSpace 3G28. Transmission DC93 initial QS1123 initial DC93 8 yrs GE QS yrs GE "top standard cell" EQE GaAs Wavelength (nm) Fig. 5. Transmission/spectral answer of a solar cell/cover glass system bounded with MAPSIL QS Spectral answer (A/W) The transmission through the sandwich is not that of the coverglass-adhesive-cell composite with anti-reflective layers in absolute value. n the other hand, the relative degradation of the cell performance will be perfectly proportionate, subject to the representativeness of the thickness: Selex Galileo announced a thickness of 7 µm, while EADS-Astrium gave a thickness of 2 µm

7 (Claus Zimmermann, Journal of Applied Physics, 13, (28))! And it seems that the tested samples show much higher thicknesses (3 to 4 µm), this point should be verified. Current densities of the cell according to the adhesive (ma/cm²) resulting from the calculation are described in Table 11. DC 935 QS 1123 Initial years GE Relative degradation -7.9 % -8.8 % Table 11. Current densities for DC 935 and MAPSIL QS Interpretations The relative degradations are high. Indeed, a degradation of.25% per year is generally considered in the predictions of power (industrial value for CMX). This degradation is partly due to the use of coverglass CM, whose UV transmission is high (wavelength cutoff : 325 nm against 345 nm for CMG and 355 nm for CMX). Very effective in begin of life, the CM are little used because of the risk of degradation of adhesives [4]. It is because we expected significant degradations that we have selected them for this study (to be sure to see something and be able to compare). In any case, the test conditions seem very severe for 8 years in GE orbit. DC 935 and QS 1123 are very similar before and after ageing. 7 6BCNCLUSIN This paper presents the qualification of the MAPSIL QS1123 silicone resin. The qualification plan was based on space applications for this resin, namely, varnishing, bonding and potting, and taking into account the means of characterization and testing available at CNES. This report describes the main properties of the resin and its performance after various ageing tests. We have summarized the main results in Table 12. Tests/Application Varnishing Bonding Potting K K Resistance to -133 C In progress -17 C temperature +116 C +2 C Resistance to damp heat K In progress K Resistance to Gamma rays (1 and 3 krad) - - K Resistance to UV + particles - Comparable to (8 years GE) DC935 Table 12. Main qualification results of MAPSIL QS1123. MAPSIL QS 1123 meets the technical requirements and give the opportunity to formulate new products thanks to flexible properties (clearness, thixotropic behaviour, thermal conductivity, electrical conductivity ). MAPSIL QS 1123 is manufactured according to IS and EN standards and all the steps are controlled by specific scientific equipment. MAPSIL QS 1123 has been developed to provide a durable alternative with regard to North American and European regulations, as well as industrial properties. REFERENCES [1] S. Remaury & P. Nabarra, Rapport de qualification résine MAPSIL QS1123, internal CNES note DCT/TV/TH/NT , 26 June 29. [2] D. Gervaud, Compte rendu d essai et de mesure de conductivité thermique de la nouvelle résine silicone MAPSIl QS1123, internal CNES DCT/TV/TH/NT9-639, 29 April 29. [3] D. Gervaud, Compte rendu d essai à pression atmosphérique de la résine silicone MAPSIl QS1123, internal CNES DCT/TV/TH/NT , 14 May 29. [4] ESA Alert in 25 on the association between CM and coverglass adhesive RTV-S695. -

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