RUBBER MATERIALS AND DYNAMIC SPACE APPLICATIONS

Transcription

1 RUBBER MATERIALS AND DYNAMIC SPACE APPLICATIONS Tony DEMERVILLE SMAC, 66 Impasse Branly La Garde FRANCE, Tel : ABSTRACT In many fields (for example automotive, aeronautic, ), rubber materials are widely used to solve dynamics problems. Since a few years, rubber dynamics parts are designed for space applications too: Historically, their damping properties were firstly used for launcher applications (ARIANE IV, ARIANE V and VEGA today), to prevent damages due to shock levels generated by stage separations. Then, these materials were mounted inside spacecrafts as passive damping equipment: Their aim is always to limit shock levels effect on onboard equipment (MYRIADE microsatellites, ATV,.) and to limit microvibrations generated by the onboard reaction wheels and gyroscopes too. For these typical devices, the main gap between launcher and satellite applications is the life time: Only few minutes for a launcher against several years for a satellite. So, it is important to know the mechanical behavior of rubber materials in space environment, in order to validate the performances of the rubber devices up to the end of the satellite life. But today the effects of space environment on rubber materials with more or less long run are not really known, because there is no real experience feedback. However, the aggressiveness of this specific environment is proven and influences the evolution of rubber materials with time. The three sources of ageing identified under these conditions are the solar irradiation, the thermal amplitudes and the vacuum. Separately, these three factors have consequences on the organic materials which are known enough, but the difficulties appear when the combination of these three parameters is considered. To understand these effects, a study started with CNES, SMAC and the MAPIEM laboratory. INTRODUCTION Rubber term is the general word used to speak about the elastomer family which belongs to the wide family of polymers. But those are specific materials, very different from standard thermoplastics or thermosets: They are very flexible, very elastic, gas-proof and thanks to an appropriately compound they can present a high damping capability. That is why they are widely used to solve dynamic problems in many fields, including space applications. Today shock absorbers and passive dampers using elastomers are mounted onboard launchers and satellites. But the effects of space environment on them are not really known still because there is no real experiment feedback, and so the questions about the lifetime are numerous. To answer these questions a study has just started to predict the behavior of SMACTANE rubber exposed in space environment during several years. 1. ELASTOMER, GENERAL INFORMATIONS 1.1 Molecular structure Elastomers are polymer materials composed of long molecular chains made of usually elements like carbon, hydrogen, oxygen,... In this elementary state, the material is strongly viscous and weakly elastic. To get its final elasticity, it is necessary to cross link these long chains together thanks to the vulcanization step using sulfur or peroxide elements. Elastomer is finally a thermoset rubber with very long molecular chains as shown the figure 1. Cross-link Long molecular Fig. 1. Molecular representation of elastomer

2 There are several families of rubber following the nature of their molecular chains, and they are usually classified in 3 categories as presented by the Figure 2. damage, and return in their original position when the stress is cancelled. Usual rubber Specific rubber Natural rubber, polyisoprene, butadiene-styrene, EPDM, isoprene, polycloroprene, Very rubber specific Silicone, polyacryliques, PU, Fig. 2. Usual classification of elastomer The polymer cannot be used only, it is necessary to include various additives to it, in order improve various properties. Here after the main list of additives usually used by the rubber industry : Vulcanizing agents and accelerators : To link the molecular chain during the vulcanization phase, it is necessary to add some vulcanizing agents, like sulfur. In a same time accelerators are added to increase the kinetic of the vulcanization reaction. Reinforcing charges : Usually black carbon is used to increase the hardness and the mechanical properties of the elastomer (breaking stress elongation, mechanical modulus). Silicate can be used too. Processing aids : To make easier the rubber process, processing aids like plasticizers are added to the compound Protecting agents: Their role is to protect the polymer from its external environment, such as UV radiations, ozone, A lot of protecting agents are available, such as organic modifiers, fire retarder, fungicides,.. Fig. 3. Space dampers during elongation test This hyperelasticity comes from the ability of the long molecular chains to reconfigure themselves in order to distribute the applied stress, and the covalent crosslinks ensure the elastomer to return to its original configuration. In addition, its mechanical moduli are very low, only few MPa. The figure 4 presents these characteristics by comparing the elongation curves obtained with an elastomer sample and with an aluminum sample. 1.3 ThermoPhysical aspect σ Aluminium Due to the high mobility of the macromolecular chains, the glass temperature Tg of elastomer is lower than the room temperature and it depends on the chemical structure of the elastomer. The figure 5 presents for a usual rubber the evolution of the dynamical modulus versus the temperature. Elastomer ε Fig. 4. Tensile test for aluminum and rubber 1.2 Mechanical characteristics The main mechanical characteristics of elastomers is extreme flexibility : They can be strongly extended (upper than 400% of elongation, see Fig. 3.) without Tg Fig. 5. Evolution of dynamical modulus vs temperature

3 When the temperature is lower than Tg, the behavior of rubber is like glass : hard and breakable When the temperature is upper than Tg, the thermal agitation is very high inside the material which becomes very elastic - Finally, if w continues to increase, molecular chains cannot return to their equilibrate position between 2 solicitations. Tensions are permanent inside the macromolecular chains, and the material seems to become rigid. The figure 7 presents the influence of the frequency on the SMACTANE 50 SP. Around Tg, this is the relaxation area where the damping of the elastomer is maximum 1.4 Viscoelasticity of elastomer Elastomer materials exhibit a viscoelastic behavior, which is between a perfect elastic spring and a viscous fluid. That is why it is usual, in first approximation, to model a rubber damper by a spring associated with a dashpot (see Fig 6.). Fig. 7. Frequency influence on SMACTANE 50 - Temperature influence : Temperature and frequency are opposite influence on the rubber : Fig. 6. Viscoelastic model When a sinusoidal exciting force is applied to such system, the strain is observed to lag behind the stress. The phase angle between them, denoted φ, is the loss angle linked to the damping capability of the rubber and the hysteresis effect. The stress σ may be separated into 2 components : one in phase with the strain (G ) and one leading it by a quarter cycle (G ) - At very low temperature, mechanical moduli are very high, the elastomer is rigid and breakable - At high temperature, mechanical moduli are low, the elastomer is elastic. In the intermediary range of temperature, the rubber is the most viscous around the glass temperature noted Tg (see Figure 8). σ=γ 0 [G (w)sinwt+g (w)cos(wt)] (1) - Frequency influence : φ=tan -1 (G /G ) As written in the equation (1), G and G depends on the frequency w : - When w is low, molecular chains have the time to return to its equilibrate position : the material seems to be flexible and elastic. Fig. 8. Frequency influence on SMACTANE 50 - When w increases, the return to the equilibrate position is in late with the solicitation ; this is the hysteresis phenomenon

4 2. ELASTOMERS AND DYNAMIC APPLICATIONS Thanks to their damping capability, elastomers are widely used to solve dynamics problems in many fields, including space applications. Firstly it was to protect sensitive equipment from pyroshock levels for launch applications. But step by step, elastomers were included inside satellite too, to deal with vibrations and microvibrations problems. 2.1 Passive shock absorbers Historically, in European space field the first shock absorbers using rubber appeared for ARIANE launchers. The aim is to protect all sensitive equipment from the shocks levels induced by the separation of the different parts and by the main release mechanism (spacecraft release, appendage deployment, ). To get there some shock absorbers were designed using rubber like SMACTANE in order to use its high damping effect. Fig. 10. Dampers used to protect an inertial unit The main principle of passive damper is linked to the behavior of single degree of response (SDOF) system (see Fig. 11.).The isolation region begins when the exciting frequency is upper than the cut off frequency of the dampers. Before the levels are amplified, but they can be limited by using a high damping rubber. Fig. 9. Example of Ariane 5 shock absorbers With such dampers, the shock levels are reduced by two means : - Firstly with the mechanical filter effect induced by the low stiffness of the isolator. - Secondly with the damping effect induced by the use of an elastomer material Thus, attenuation upper than 40 db can be expected at high frequencies 2.2 Anti vibrations dampers During the launching phase, satellites and their equipments contend with a critical vibratory environment, which can damage definitively the onboard equipments. To protect them a viscoelastic dampers based on elastomer can be used as passive solution. Fig. 11. Typical transmissibility curve 2.3 Micro vibrations dampers Today to no disturb the accuracy of the onboard optical instruments, it is often necessary to damp the microvibrations generated by some of equipments such as reactions wheels, gyroscope, compressor, A good solution is to isolate disturbing equipments from the satellite structure with a viscoelastic damper. The efficiency is always based on the SDOF behavior but in this case the exciting source comes from the isolated equipment.

5 the ageing mechanism for an elastomer exposed to space environment. Three sources of ageing were identified today for elastomer materials exposed to space conditions: - The solar irradiations - The thermal amplitudes - The vacuum Fig. 12. Example of micro-vibration dampers 2.4 Structural damping The last application of rubber material is to bring damping structural and so to limit amplification modes of metallic parts, which can disturb the global system. To get there the rubber can be directly bounded on the part to damp. These three factors have consequences on all organic material, and influence the evolution of the rubber mechanical properties with time. Generally speaking, to deal with ageing problems of polymer, the most frequently used tools is the Arrhenius model which describes the velocity of one chemical reaction versus the temperature. Thus, for example, the kinetic of oxidation reaction inside the natural rubber can be predicted and the evolution of its rigidity can be predicted against the time. But about the space ageing, the using of Arrhenius gives only a roughly estimation, because there is not only one ageing mechanism but three, and moreover the combination of these three parameters have to be considered. In addition, there is no real feedback around all these questions: today no elastomer samples were analyzed after having spent time in orbit. Fig. 13. Links damped with SMACTANE rubber 3. ELASTOMERS AND SPACE ENVIRONMENT 3.1 Aggressiveness of space environment on rubber material As shown in the previous paragraph, rubber materials are interesting to reduce dynamic levels that it is with shock absorbers or vibration/microvibrations absorbers. Contrary to the launcher applications, the question of rubber ageing becomes important for all satellite applications with several years for life time. Indeed, it is necessary to validate the performances of rubber devices up to the end of the satellite life, and so to know So to understand better these ageing mechanisms inside rubber, a complete study cofinanced by CNES have just started with the MAPIEM laboratory, and it is based on SMACTANE rubber. Firstly the study will begin with a complete tests campaign to know the effect of each individual ageing source. And then the effects of their combination will be studied, to finally predict the mechanical evolution of the rubber exposed to space environment during 10 years. Hereafter the first results of this study which started with vacuum effect. 3.2 Vacuum and rubber outgassing The first effect of vacuum on elastomer is to release volatile molecules when it is exposed to high vacuum, and so they are not designed for space applications

6 because exceed the acceptance limits laid down in the ECSS-Q-70-02A (see figure XXX) Neoprene elastomer Nirile elastomer Polyurethane elastomer TML % RML% CVCM% Fig. 14. Outgassing value of standard rubber To improve these data, the first way is to apply a pre vacuum bake to decrease significantly the outgassing values below acceptance limits, as shown the following results obtained with SMACTANE rubber : Before vacuum bake Afetr vacuum bake ECSS-Q A This experiment shows it is possible to use elastomer in compliance with ECSS out gassing requirements by using specific compounding. 3.3 Vacuum and mechanical effects on rubber The first idea is to think the ageing of rubber should be better in vacuum due to the absence of oxygen. But this idea is wrong because vacuum brings some degradations inside the elastomer structure, because of several origins : Evaporation of light components Desorption Break of molecular chains To evaluate the mechanical effects of vacuum, a comparative study was performed on SMACTANE, using different vaccum conditions ( room pressure, primary vacuum and secondary vacuum) with the same thermal conditions. TML % RML% <1.0 CVCM% <0.1 Fig. 15. Pre vacuum bake effects on SMACTANE However the validity of this procedure is only demonstrated under µvcm experiments, where rather small sample than 2mm. And when a significant amount of rubber is enveloped by e metallic housing like a damper is used, a large amount of volatile material is still present in the core of material after the pre treatment. Fig. 16. View of samples during ageing tests Among the different mechanical tests performed on the aged samples, tension tests show the most important evolution of the rubber. Compared with the samples exposed to room pressure, the tension modulus of rubber increases significantly (+14%) after 14 days vacuum exposure (see Fig. 15). Finally the best solution is to work directly on the compounding of the rubber, to limit the amount of volatile material. Thus, after the identification of critical compounding materials into the SMACTANE, they have been substituted, treated and modified and a new formulation was created with very low out gassing rate : The SMACTANE SP (RML 0.31%, CVCM : 0.02%). Fig. 17. Effect of vacuum on SMACTANE tension modulus

7 On the other hand there is a low gap between the effects of primary and secondary vacuum. But these results can be discussed because tests were not performed on the outlet side of the vacuum facility, and so some hypothetical reversible reactions were not taken into account CONCLUSION Finally rubber materials can be adapted for space applications and particularly to solve dynamic problems. To use elastomer parts, it is enough to not forget that their behavior depends on temperature, frequency, and mechanical levels. That is why the environment (mechanical and thermal) has to be specified accuracy, and elastomer parts have to be associated with a range of performances and not with just one performance. Today the main progress way is to understand the complex ageing mechanism of these materials exposed to space environment, in order to validate their using for long time missions. To get there a study has just started and will be carried out up to 2012, to finally predict the behavior after a long time of an essential material : the rubber.