Radiat. Phys. Chem. Vol. 35, Nos 1-3, pp. 204-212, 1990 Int. J. Radiat. Appl. Instrum., Part C Printed in Great Britain

0146-5724/90 $3.00+ 0.00 Pergamon Press plc

RADIATION DURABILITY AND FUNCTIONAL RELIABILITY OF POLYMERIC MATERIALS IN SPACE SYSTEMS

Y. HARUVY

Soreq Nuclear Research Center, Yavne, 70600, Israel

ABSTRACT

Polymeric materials are preferred for the light-weight construction of space- systems. Materials in space systems are required to fulfill a complete set of specifications, at utmost reliability, throughout the whole period of service in space, while being exposed to the hazardous influence of the space environment.

The major threats of the space environment in orbits at the geostationary altitude (GSO) are implied by the ionizing radiations, the main constituents of which are highly energetic protons (affecting mainly the surface) and the fast electrons (which produce the main threat to the electronic components). The maximum dose of ionizing radiation (within the limits of uncertainty of the calculations) at the surface of a material mounted on a space system, namely the "Skin-Dose", is ca. 2500 Mrads/yr. Space systems such as telecommunication satellites are planned to serve for prolonged periods of 30 years and longer. The cumulative predicted dose of ionizing-radiation over such periods presents a severe threat of chemical degradation to most of the polymeric construction materials commonly utilized in space systems.

The reliability of functioning of each of the polymeric materials must be evaluated in detail, considering each of the relevant typical threats, such as ionizing-radiation, UV radiation, meteoroides flux, thermal cycling and ultra-high vacuum. For each of the exposed materials, conservation of the set of functional characteristics such as mechanical integrity, electrical and thermo-optical properties, electrical conductivity, surface charging and outgassing properties, which may cause contamination of neighboring systems, is evaluated. The reliability of functioning of the materials exposed to the space environment can thus be predicted, utilizing data from the literature, experimental results reported from space flights and laboratory simulations, and by chemical similarity of untested polymers to others, which are widely experienced for space uses.

KEYWORDS

Radiation Durability; Polymeric Materials; Space Systems; Reliability; Space Environment; Degradation; Outgassing.

INTRODUCTION

Reliability Reauirements from Polymeric Materials Selected for Space Systems

Space systems are characterized by a set of unique parameters which imply the extremely strict reliability requirements: a. space-systems are extremely expensive to develope due to complex set of

functional reqirements. b. there is a strict limitation of weight, enforcing the engineer to apply

uncommon techniques and complex packaging. c. each "product" is manufactured only once, or few times at most. d. there is no possibility for maintanance operations in orbit, except for

few systems in manned vehicles.

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7th International Meeting on Radiation Processing 205

e. a failure in a space-system operation may lead to failure of a whole mission, which corresponds to a multimillion damage cost.

The above stated parameters set forth the ensemble of reliability requirements as well as the set of operative measures required to attain the desired functional performance of the polymeric materials in space systems. However, in the ordinary attitudes of reliability assessment, namely in the analysis techniques such as FTA, FMECA, WCA" etc., relatively little caution is paid to the problematics of the possibility of a functional failure of materials during the operation of the system. Hence, the selection of polymeric materials for space applications is seldom as cautious as the selection of other constituents of the system, such as mechanical and electronic components.

The Durability of Polymers in the Snace-Environment: A Basic Hypothesis

In the commonly used practice of assigning polymeric materials to tasks in space systems, two assumptions are being concealed: first, the attitude that if a polymer which is utilized to build the system is commonly used for similar applications, it will most probably fulfill faultlessly all the functional requirements. Secondly, there lies the hypothesis that no drastic changes are implied during the service period to materials which are "known to be durable to the space environment". Hence, the initial functional performance of these materials is blieved to be reliable throughout the entire mission in space.

These two hidden assumptions may already be erronous to some extent in many terrestrial systems. They are radically wrong for space sytems, since the disconsidering of the change of functional reliability of the polymeric materials upon exposure to the hazards of diverse space environments may lead to disastrous faults. Both hidden assumptions are wrong for two reasons: first, there exists very little published data regarding the functional reliability of materials in space systems. The reasons are either secrecy classification of the mission or commercially motivated secrecy. Secondly, the nature of the space environment is completely different from terrestrial existing environments, both in harshness, diversity and complexity. Therefore, an implicit methodical and thorough consideration of each polymer selected for incorporation in the space system is a must. Furthermore, the analysis of the compliance of a polymer to a specific task and environment must be established on a basic hypothesis that every material is apt to fail, unless a sufficient proof is given to its durability to every single hazard of the space-environment, to which it will be exposed during the service period in space (and during terrestrial storage).

In the present communication, the radiation hazard to polymers in the space environment and the systematic framework for the selection of materials for space systems, as well as predicting their functional reliability before and during the service period will be described.

SPACE-ENVIRONMENT THREATS TO POLYMERIC MATERIALS

The polymeric construction materials are widely utilized in space-vehicles and satellites due to their high strength-to-weight ratio [i-2], since light-weight construction is a major factor in these systems. However, the polymeric materials are the most sensitive to the environmental hazards of the outer space, especially to the ionizing radiation. The materials mounted in the envelope regions of the satellite are being exposed to the imperiling influence of the space environment in its utmost harshness. Materials in the inner parts of the satellite will be exposed only to part of the hazards, and only to reduced intensities of them. The hazardous environment comprises various types of ionizing radiations, fast-particle fluxes and thermal fluctuations, as follows. a. ionizing radiation: comprises fluxes of energetic electrons and protons the energy of which ranges up to few MeV, for both the electrons and the protons [3]. The predominant constituent of the ionizing radiations in GSO, regarding the hazardous influence on the outermost surface-layers is the flux of the protons, which are mostly stopped at these layers. b. atomic oxygen: (ATOX, relevant only to low orbits, e.g. the shuttle orbit

" FTA, FMECA and WCA denote Fault Tree Analysis, Failure Modes Effects and Cryticality Analysis and Worst Case Analysis, respectively.

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206 Y. H ~ u v v

altitudes of ca. 200 km) neutral O atoms collide the front surface of a space-vehicle at a relative velocity of about 8 km/s which corresponds to energy of about 5eV [4]. c. UV radiation: a constituent of the albedo sun-light of intensity of ca. 37~ of the UV sun illumination intensity [5]. d. temperature cycling: caused by the exposure of the surfaces of a space- vehicle to more than 104 cycles, of sun-light and darkness, during a 30 years mission [5] in the GSO. e. high vacuum: values of 10 -9 to 10 -11 are quoted for the vacuum in the outer-space, and a lower vacuum of about 10 "s for the satellite's surroundings.

The major hazard to polymers, in the space environment at the GSO altitude is the ionizing radiation. Unlike the radiation sources commonly used for the radiation processing, the radiation sources in space are of various types: fast electrons and protons, ~ radiation and energetic heavy particles. All these ionizing species come forth in a wide spectrum of energies, the fluxes at each one diverge significantly from one orbit to another (depending on the latitude). These various radiation sources are schematically described in Fig. i. The fluxes of the two major constituents of the ionizing radiation, namely, the electrons and the protons at orbits of various distances from earth are displayed in Fig. 2 and Fig. 3, respectively.

Space Radiation Sources

GALACTIC ANn

EXTRA-GALACTIC SOURCES

SOLAR WIND - ~,. ~ . / /

e- 9' NEUTRONS % . " " . . . . . . . . . . o~nTnu~ cTr" ~ L;U~IYIIb MAT~ " '?" . . . . . . ~ '~" " - - - " ~ IHEAVY IONS]

EARTH'S MAGNETIC FIELD

ORBITAL l ' { -- I ~ DEPE.0ENCE

~ _ _ _ J ) \~__./

Fig. i. The space radiation sources around earth

RADIATION SKIN-DOSE AND SKIN-DEGRADATION OF POLYMERIC MATERIALS

The ionizing radiation in space is characterized by a wide variety of constituents, the major part of which are fast electrons of energies varying from few eV to several MeV. In the GSO, the main constituent of the radiation is the flux of fast protons, most of which is stopped at the outermost regions of the impinged material. A typical energy distribution of the ionizing radiation of the protons in the GSO altitudes is as follows: -40~ - particles of energy less than 20keV, -45% - particles between 20keV and 0.SMeV and -5% - particles having energy higher than 0.SMeV [3]. For the highly energetic electrons the energy distribution is as followes: -40~ - particles of energy less than 50keY, -40% - particles between 50key and 200keV and -20~ - particles having energy higher than 200keV.

7th International Meeting on Radiation Processing 207

3

2

!

w

I

2

E L E G T R O N a / O M a - - B E C

-- E N E R G Y ~ 0 . 0 4 M e V

Fig. 2. The trapped electrons belts around earth

2

I

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2

PROTONS/CM'-SEC

10 10'

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oo7 J))1 7 " ' "

10 I0' Energy >_ 1.0 MeV

Fig. 3. The trapped protons belts around earth

The estimate of the energy distribution of these fast electrons and protons in the GSO environment is established on an in-flight experimental data [6], accumulated by applying various methods of dosimetry techniques. However, most of the data available today refers to electrons of energies above 40keV, while ignoring the less energetic ones, most probably due to the limitations of the existing dosimetry techniques and equipment. Similar reasons bring about the lack of data regarding the protons of relatively low energy (less than few tens keY).

The most essential dose calculation is that of the "Radiation Skin-Dose" (RSD), namely, the calculation of the effective degradative dose for the outermost, unshielded layers of a polymeric component. One should remember that we are dealing with a multi-energy spectrum of radiation species. Hence, it is evident that such calculation must take into consideration almost all the spectrum of energies of the ionizing radiation species, since the average bond-dissociation-energy in the organic molecules is about 4 to 5eV. In other words, since breaking a chemical bond in a polymeric chain requires only ca. 5eV, the effect of all the impinging species of energies higher than few eV must be accounted.

208 Y. HnRLrW

Such dose calculation is not relevant for the inner layers of the polymeric material (e.g. at a depth of 0.Smm ), since the less energetic species of the ionizing radiation spectrm can not penetrate into the depth of the impinged material. Therefore, the existing calculations of the dose-depth profiles are valid, except for the outermost regions of the exposed material. For the latter regions, ignoring the low-energy fraction of the radiation leads to a severe error in the calculated dose. Furthermore, the relatively low-energy species deposit all their energy in the short range of the outermost layers and hence, effect a most considerable contribution to the skin-dose.

From the data of the abundance vs. the energy of the particles in the GSO [6] it is known that the relative number of the low energy species may be higher by several orders of magnitude than that of the high-energy ones. Hence, it is obvious that the disregarding of the lower-energy part of the electrons and protons spectra of the ionizing radiation in space induces a severe underestimation of the radiation threat for the "skin" of polymeric materials. It should be pointed out that the terminology dose used herein refers to a distinct period of service in space and actually corresponds to a dose rate. A typical dose-depth profile in the GSO, which takes into consideration the "Skin-Dose" is shown in Fig. 4.

1000

100

: E

O

O 10

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- - - - o \

I " \

0.1 1 10 100 1000 10000

Depth (gm Kapton) (60 Rad / year)

t h e " S k i n - D o s e " Fig. 4. A Dose-Depth Profile in KAPTON, Showing Effect. Dose is Calculated using the Half-Space Model (3) . Calculated for the GSO, 0 ° Incl 160 ° W (PARK).

It is self evident that the higher the skin dose-rate is, the faster and more intense are the processes of degradation at the skin-layer of the polymer, followed by erosion and cracking, which tend to propagate inward the bulk of the material. Furthermore, a salient product of this multi-bond-breaking process is the radiation induced outgassing inventory, as disdussed in the following. Hence, the underestimation in the dose rate that may reach several orders of magnitude will manifest, in the worst case, in a proportional decrease of the fault-free mission period or, in other words, a severe mal-functioning of the systems much earlier than expected.

THE SELECTION OF MATERIALS FOR APPLICATION IN SPACE SYSTEMS

The polymeric materials are selected for use in space systems from the huge collection of commercially available materials, unless a special "space grade" material is quoted for such uses. It is noteworthy that conservativity in the space industry rules out almost any newly developed material unless there exists an extensive experimental proof to its long-term performance and environmental durability.

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Polymeric materials for space uses can be generally classified into three convenient groups, according to their durability to radiation: highly sensitive polymers, stable polymers and highly-stable polymers, which correspond to damage thresholds of less than I MRad, 1-100 Mrads and higher than 100 MRads, respectively.

Polymers of each group can be assigned to tasks in different regions of the space system, according to the intensity and combination of the space- environment hazards. Only solid-like polymeric materials are comprised in this classification, since liquids and pastes are not allowed in space-systems (except in sealed compartments), to prevent outgassing and contamination problems, as will be discusssed in the following. Typical polymers belonging to the above stated classes are DELRIN (polyacetal) and TEFLON TFE (highly sensitive), KYNAR (PVDF) and TEFLON FEP (stable) and KAPTON (polyimide) and PET (highly-stable).

It should be stressed that chain scisioning of the polymers is not the only mechanism leading to functional failure. Radiation induced crosslinking and change of the flexibility characteristics of a polymeric component can also be malefic when these properties are critical. The classification of polymers according to their durability in space is further complicated by their different sensitivity to UV, thermal cycling and ATOX (if relevant). One generalization, however, is found very useful for the radiation-durability classification of polymers: the more aromatic the backbone is, the lesser the radiation induced changes are. It should also be mentioned that the non-aromatic siloxane polymeric-backbone exhibits a remarkable tolerability to ionizing radiation.

One of the early stages of building a space system is the assigning of specific polymers to definite tasks. At this step it is essential to eliminate the incorporation into the space system of polymers which are apt to be deteriorated. This step is preferrably performed with the support of a data-base of properties and environmental durability of materials for space applications. The accumulation, compilation and utilization of such data-base is described elsewhere [7]. Thus, for every polymer the threats of the environment of the specific mission orbit and the desired mission duration have to be carefully evaluated, regarding explicitly the exact region of the system where the polymer is planned to function. Hence, to try to predict any possible long-term environmental induced degradation which may threaten the performance of each polymeric material at each region of the satellite.

THE OUTGASSING PLAGUE OF POLYMERS

Polymeric solid matrices usually comprise molecules of ultra-high molecular weights (typically several hundreds of thousands daltons). Polymers contain, however, volatile compounds originating either from low molecular weight fractions of the polymer itself, or from trace impurities of the manufacturing and the processing hystory of the specific polymeric material and the specific batch of the product. These volatiles can diffuse through the polymeric matrix and migrate towards the surface of the polymer. During the service period in the space environment, these volatile ingadients evaporate, migrate through the vacuum surrounding the systems of the satellite and eventually part of them settle and fixate elsewhere in the space vehicle, causing a mal-functioning of sensitive systems such as solar-panels, sensors, optical components, etc.

Hence, the desired mechanical integrity, strength and flexibility performance of polymers adopted for space applications must be accompanied by non-outgassing characteristics, which are crucial to ensure reliable constant thermo-optical properties of the envelope materials of a satellite, as well as to retain reliable functioning of all contamination-sensitive systems during prolonged service in space, where no maintanance and almost no corrective actions can be performed.

The initial characteristics of the outgassing of a polymeric component in a system is an outcome of its chemical nature, namely, its molecular weight distribution (the weight fraction of low M.W. fragments), and to a lower extent, its chemical stability. However, the outgassing inventory is strongly affected by the manufacturing processes of the specific batch in use, as well as the various shaping, workmanship, curing, finishinf and even cleaning (!) processes which this polymeric component undergoes prior to its mounting on a space vehicle.

210 Y. HARUVY

These outgassing characteristics can be measured by the standard method of the ASTM E-595 which is worldwide accepted. The results of this test method [8] are interpreted in terms of %TML - the total mass loss of the sample, and the %CVCM - the collected volatile condensed mass. If desired, the quantity of the TML and CVCM inventory can be significantly reduced by a distillation processing of the precursor resin of the polymer (e.g. paints, adhesives, potting compounds, etc.).

Alternatively, the outgassing inventory can be reduced by a "bake-out" processing of the finished product [9], in a vacuum chamber, at elevated temperatures. Usually, such process can bring most of the polymeric materials to outgassing characteristics wich pass the standard upper allowable limit of I% TML & 0.1% CVCM [i0].

This measurable and melioratable inventory of outgassing compounds we define herein as the "Virgin Outgassing" (VOG). Values of VOG are regularly measured and are reported for numerous polymeric materials and are tabulated in various data-banks and databases for space materials [11-13]. However, for most of the polymers this information regards the less-severe obstacle for their space utilization, since this blemish is the one which can easily be taken care of. Conversly, fragmentation of polymers by the hazards of the space environment, namely, the ionizing radiation and the ATOX (in low earth altitudes), will most probably cause the built up of a new and much more dangerous inventory of outgassed materials. At altitudes above I000 km, and GSO in particular, the radiation induced outgassing is predominant, and is further amplified due to the Skin-Dose mentioned above.

For these hazard-induced types of outgassing we use hereafter the terminology "Radiation Induced Outgassing" (ROG) and "ATOX induced Outgassing" (AOG). Both ROG and AOG are seldom discussed in the literature. No quantitative data, measured either in laboratory or during in-flight experiments in space can be found in the open literature. The severity of the adversity of the induced outgassing inventory is furthermore enhanced by two factors: first, by the fact that the induced outgassing inventory is composed mainly of fragments of the eroded macromolecular polymeric materials. These fragments are of relatively high molecular weights and add, most probably, to the CVCM inventory, the outgassed fraction which is contaminating and hence, it is hazardeous for the functional reliability of other systems.

Secondly, these fragmented molecules are produced at the outermost surface (the "Skin") and its close vicinity. Hence, the outgassing course is not hindered by diffusion processes and the outgassed molecules can readily migrate towards the contamination-sensitive sites. Hence, the relevant hazardous inventory of the outgassed materials is the sum of the above described three contributions, namely, the "Total Outgassing" (TOG):

(I) TOG = VOG + ROG + AOG

Furthermore, unlike the VOG, both the ROG and the AOG contributions to the TOG inventory can neither be measured nor eliminated prior to the service period. It is most important, therefore, to try to evaluate the potential degradative effects of the environmental hazards, each one by itself and in combination. Further, to try to estimate the particular damage for specific polymeric candidate materials in space systems, and to try to predict the potential induced outgassing inventory produced thereby.

THE SYSTEMATIC METHOD FOR THREATS EVALUATION, FAILURE MODES AND EFFECT ANALYSIS FOR POLYMERIC MATERIALS IN SPACE-SYSTEMS

Fig. 5 presents the approach and tools for the systematic evaluation of the interaction of the environmental threats with the materials constructing the space system. This approach is demonstrated via the analysis of the exposed surface of an electronic box. This type of analysis is performed for each system or sub-system which are exposed to the space-environment. Emphasis is given to analyze systems in the outermost regions of the satellite, and especially to the materials employed in the envelope of these systems.

In the first stage we list each one of the materials, from the outermost one inwards, specifying the thickness of each layer (thus determine the radiation dose of the next layer). It is noteworthy to mention that unlike polymers, metals and ceramics are tolerable to the hazards of the space-environment, with a few exceptions (e.g. silver, metal-sulfides, low-purity glass, etc.).

7th International Meeting on Radiation Processing 211

In the second stage the hazardous interaction of each threat with each layer of material is qualitatively rated, from very-severe to very-slight ( cf. the left half of the Table in Fig. 5). The third stage includes qualitative evaluation of the intensity of the various damage routs {cf. the right half of the Table), which may follow the effect of the threats on the materials (each threat by itself as well as by their synergistic effects).

EVALUATION OF THREATS TO MATERIALS, DAMAGE ROUTS AND FAILURE MODES

SYSTEM #:01.Ol NAME OF THE SYSTEM: Electronic Box PLAN VERSION #: Ol SYSTEM LOCATION: Pendant (GSO) EVALUATION #: 03 CONNECTED SYSTEMS: Sensors PREPARED BY: YH DATE: 1.8.88

No. PART NAME

1 Paint (black)

2 Primer

P r i m e r

Box Wall

MATERIAL I~R~FATS DAMAGE & FAIL51%E ID~qTIF. RI AO UV TC HV N~ HI PU ~ aa EE EC

Z-~e6 . ÷ . . . . . . . . . . . . ÷- ~O~m thick

P-128 Epoxy . . . . . . . 5O~m thick

BE-127 ............................. 20~m thick

A1 6065 ............................. n t h i c k

REM SC

++ Outer layer (1)

- - (i)

(1) (2)

RI - IONIZING RADIATION MI - MECHANICAL INTEGRITY A0 - ATOMIC OXYGEN PU ' PUNCTURING UV - ULTRA-VIOLET LIGHT OG - 0UTGASSING TC - THERMAL CYCLING ua - ABSORBANCE (two sides) HV - HIGH VACUUM EE - EMISSIVITY (two sides) MF - METEOEOIDE FLUX EC - ELECTRIC CONDUCTIVITY

SC - SURFACE CHARGING REMARKS: F~ = F~ERI~NTAL EVALUATION RECOMMENDED

CHG = CH~GE OF MA~_RIAL IS RECOMMENDED

T~T/DA~GE RATING ++ - VERY SEVERE + SEVERE +- MODERATE - SLIGHT -- VERY SLIGHT ? UNKNOWN

(1) - palntSurfaCeorSh°uldthe primer.be extremely clean and dry prior to application of the

(2) Shelf life of cured primer is 1 month, unless the item is packed. (3) -

Fig. 5. Framework for the systematic evaluation of threats, failure modes and effect analysis for the polymeric materials in space-systems

The last stage of the analysis involves the assignment of recommendations for the designer. Such recommendations include the following:

a. replacement of a sensitive material by a highly-durable one which comprise similar functional characteristics.

b. experiments, involving the exposure of the material to a simulated threat, to allow quantitative evaluation of the extent of the expected damage.

c. warnings regarding processes such as painting, potting, application of adhesives, etc. Correct performance of such processes is crucial, in all probability, for the space-environment durability of process-sensitive polymers.

212 Y. HARUVY

CONCLUSIONS

The polymeric materials in space systems must tolerate the harsh environment of space. Assurance of their reliable functioning is crucial for the successful completion of the satellites missions. Broad knowledge of the sensitivity of polymers to the ionizing radiation in space, as well as of the other hazards, namely, UV, thermal cycling and ATOX, is essential, and special attention must be paid to the polymers which are exposed to the synergistic effects of all these hazards. Polymeric materials should be carefully selected, conscientiously assigned to specific tasks and correctly processed. All this, in order to eliminate degradation of their performance during the entire period of service in the space-environment and thus, to contribute to the assurance of the desired reliability of the space system.

REFERENCES

[ 1] G Lubin and S. J. Dastin, "Aerospace Applications of Composites", Chap. 28 in "The Handbook of composites", G. Lubin, Ed., Van-Nostrand (1982).

[ 2] R. Staunton, "Environmental Effects on Properties of Composites", Ibid, Chap. 19 (1982).

[ 3] M. Israely and Y. Shamai, Data-Bank for Radiation Fluxes in Space (unpublished), Private Communication (1988).

[ 4] J. Dauphin, "Atomic Oxygen - A Low Orbit Plague", in "Looking Ahead for Materials and Processes, J. DeBossu et al., Eds, p. 345, Elsvier Publ. ( 1 9 8 7 ) .

[ 5] C. G. G o e t z e l , J . B. R i t t e n h o u s e a n d J . B. S i n g l e t a r y , " S p a c e M a t e r i a l s H a n d b o o k " , A d d i s o n - W e s l e y ( 1 9 6 5 ) .

[ 6] "Computer Codes for Space Radiation Environment and Shielding", WL-TDR-64-71 (1964).

[ 7] Y. Haruvy, "Database of Materials Properties, Technical Data and Durability Predictions for Space Applications", Presentation no. 7.6.4, The 7th. Int. Conf. Israel Society for Quality Assurance, Tel-Aviv, Nov. 8-10 (1988).

[ 8] "Total Mass Loss and Collected Volatile Materials from Outgassing in a Vacuum Environment", ASTM E-595 (1983).

[ 9] J. J. Scialdone, "Optimization of Outgassing Bake-Out Temperatures and Duration of Space Systems ", J. Environ. Sci., 29, 38 (1986).

[10] Soreq NRC Outgassing Testing Lab, Experimental Data Files (unpublished) (1987-1988).

[Ii] "Outgassing data for selecting spacecraft materials", NASA-RP-II24 (1987).

[12] A. Zwaal, Periodic Data Sheets on Co-Ordination of Materials Studies, TQM/ESTEC (1986-1988).

[13] "Outgassing and thermo-optical data for spacecraft materials" MATLAB 002 (1987).