Photopolymerisation of composite material in simulated free space environment at low Earth orbital flight

EUROPEAN POLYMER JOURNAL

European Polymer Journal 42 (2006) 2703–2714

www.elsevier.com/locate/europolj

Photopolymerisation of composite material in simulated free space environment at low Earth orbital flight A. Kondyurin a

a,b,*

, B. Lauke b, R. Vogel

b

Applied and Plasma Physics, School of Physics (A28), The University of Sydney, NSW 2006, Australia b Institute of Polymer Research Dresden e.v., Hohe Str. 6, Dresden 01069, Germany Received 25 November 2005; received in revised form 26 April 2006; accepted 29 April 2006 Available online 16 June 2006

Abstract Inflatable Gossamer’s structures which can be deployed in Earth orbit have great prospects in the future. Material for use in inflatable structures must be soft before and during unfolding and harden after unfolding. The best way for solidification of Gossamer’s structures is by chemical polymerisation of a composite material in space environment. The polymerisation processes of liquid polymer matrix in a free space environment are sensitive to microgravitation, temperature variations (150 to +150 C), high vacuum (103 to 107 Pa), atomic oxygen flux (in LEO), UV and VUV irradiations, X-ray and c-irradiations, high energy electron and ion fluxes. In this paper experiments of the photopolymerisation processes under simulated free space conditions were carried out. The influences of high vacuum, temperature variations, high energy ion beam, radio-frequency and microwave plasma on polymerisation of UV-curing resin were observed. The effects of low molecular components evaporations, acceleration of curing kinetics, additional chemical reactions and mixing processes during polymerisation were observed. The photopolymerisation of inflatable structure was carried out under simulated temperature conditions of space flight in low Earth orbit. The results indicate a technology for large-size inflatable constructions in Earth orbit, in far space and on space bodies as for deployed antennas, solar sail stringers, solar shield stringers, frames for large-size space stations, frames for Moon, Mars, asteroids bases and frames for space plant on Earth orbit and other celestial bodies.  2006 Elsevier Ltd. All rights reserved. Keywords: Composite; Photopolymerisation; Space environment; Inflatable structure

1. Introduction: Inflatabled large-size space constructions The future of space exploration will be dominated by the use of large-scale constructions in *

Corresponding author. Address: Applied and Plasma Physics, School of Physics (A28), The University of Sydney, NSW 2006, Australia. Tel.: +61 2 93515962. E-mail address: [email protected] (A. Kondyurin).

space. These can include high aperture antennas, solar sails, solar shields and large-size space stations in Earth orbit, Moon, Mars and asteroid bases. At present the size and mass of space constructions sent from Earth are limited by the capacities of the launch vehicles. A large-size construction in space can be created by the technology of polymerisation of fibre-filled composites and a reactionable matrix. For the construction frame, fabric impregnated with a long-life

0014-3057/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2006.04.018

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matrix (prepreg) is prepared in terrestrial conditions and after folding, can be transported in a space ship to orbit and kept folded on board. In due time the prepreg is transported to free space and unfolded by inflating an internal hermetic shell. The reaction of matrix polymerisation then initiates. After complete reaction, the artificial durable frame can then be filled with air, apparatus and life support systems. The concept of large-size construction in space has evolved through a long history of inflatable space structures, ageing experiments with polymeric materials in low Earth orbit (LEO), investigations of the polymerisation process in gel under microgravity condition on board of space stations and experiments of the polymerisation process under simulated free space conditions in laboratory. The development of inflatable structures for space application began with the ‘‘Echo’’, ‘‘Explorer’’, ‘‘Big Shot’’ and ‘‘Dash’’ balloon satellites [1] in the 1960s. Based on the success of these balloon satellite flights, new projects of inflatable structures based on light polymer films for antennas, reflectors, Lunar and Mars houses, bases and airlocks have been proposed [2– 5]. Since then, inflatable structures based on new and unique materials have been developed for space applications [6–11]. The world leaders of space inflatable structure production are American companies such as ILC Dover and L’Carde, Inc., which are supported by NASA [11]. However, inflatable structures have not achieved wide application in space because of high risk damage to the inflatable shells. To increase the durability of inflatable structures, the method of rigidization can be used after deployment of the structure in space. The following methods of rigidization are currently under consideration: rigidization of soft polymer matrix due to chemical reaction with thermal initiation, with UV-light initiation and with initiation by inflation of catalytic gas; mechanical rigidization due to the stressed aluminium layer of the deployed shell; foam inflation; passive cooling below Tg; and evaporation of liquid swells from gels [12–20]. In some cases a combination of hard and rigidizable structures was developed [21,22]. All of these methods have been tested in Earth laboratory experiments. Only the aluminium stressed layer method has been tested under real space conditions [2,23,24]. The free space experiments during space station and satellite missions have been carried out with solid polymer materials. The effects of the free space

environment on the polymer materials were analysed during and after flights when the polymers were exposed to atomic oxygen, VUV light, X-rays, electron and ion flows, thermal cycling and high vacuum (SETAS, LDEF, MEEP, SARE, AORP, DSPSE, ESEM, EuReCa, HST, MDIM, MIS and MPID missions). The experiments with the polymerisation processes in Earth orbit were carried out inside space stations under the influence of microgravity only [25]. The conditions of free space have destructive effect on solid polymer materials. In free space polymer materials are exposed to high vacuum, sharp temperature changes, space plasma formed by deep space rays, sun irradiation and atomic oxygen (in low Earth orbit), micrometeorite fluency and microgravitation [25–35]. The European Space Agency has recently resumed research activities in the field of inflatable structures in the frame of the Technology Research Programme. The studies are on-going in order to demonstrate the feasibility of deploying very large appendages and structures. These studies include design, manufacture and testing of elementary inflatable items and the development of an in-flight demonstrator. Preliminary studies have concluded that the long durability of the construction in space environment requires rigidization after inflation. The approach believed to be most promising is polymerisation of the composite material in free space environment. The thermal and photopolymerisation processes have been considered. The effects of the space environment before, during and after curing are important and must be taken into account in the design for both ways of the polymerisation process. In previous work [34] the thermal initiated polymerisation process in simulated free space environment was done for epoxy resin compositions as well as prepreg based on carbon and glass fabrics. The influence of simulated space environment on the polymerisation processes, structure and properties of composite materials has been examined. In this paper we consider the simulated space environmental effect on the photopolymerisation processes and structure of UV-curing polymer matrix. Such matrices are proposed for the rigidization of inflatable structures in space. At present, Alcatel Space, EADS Space Transportation and other companies develop inflatable space constructions based on rigidizable polymer matrix under irradiation from UV lamps. Such lamps will operate on the Earth orbit to initiate the photopolymerisa-

A. Kondyurin et al. / European Polymer Journal 42 (2006) 2703–2714

tion reaction in the inflated construction on command. In our study we used UV-curable resin for space demonstrator which is planned to be cured on the Earth orbit in future. 2. Experiment The experiments of photopolymerisation in simulated free space environment were carried out using UV-curing resin R36-43Xi and R43 R53 R36/38 components mixture (Green Isolight International, France). Glass textile (Interglas, 01623-110-2052 kind) of 95 g/m2 density was used for reinforcement. This composite was developed for the rigidization of inflatable structures at European Space Agency missions in LEO. UV-lamp Blak-Ray, Model B 100 AP (UVP, Upland, USA) and UV2M unit (BBH Ltd, England) with two TL8W/05 lamps (Philips, Holland) were used as UV light sources for photopolymerisation of the resin. The digital radiometer DRC-100X (Spectronics Corporation, New York, USA) with electro-optically calibrated sensors DIX 365A and DIX 300A was used to measure the UV light flux. The UV-light power stability and geometric distribution was monitored during experiments. UV power on the sample surface was maintained in the 100–10,000 lW/cm2 range. Both lamps have a Hg spectrum, with the most intensive line at 365 nm in the UV region. The plasma treatment was carried out in a microwave plasma reactor with a low pressure discharge. The plasma reactor 440G (Technics Plasma GmbH, Germany) with 2.45 GHz plasma frequency, 200 W plasma power with homogeneous distribution in a 23 l volume reactor chamber, 12 Pa pressure of oxygen with a flow of 10 sccm was used for plasma discharge. The temperature of the samples increased to 40 C during 1 h of plasma treatment. For ion beam treatment the samples were treated by plasma immersion ion implantation (PIII). The nitrogen plasma was generated by a radio-frequency plasma source with the density of 1010 cm3 and an electron temperature of a few electron volts. After the sample was placed in the chamber, the chamber was evacuated to 103 Pa for 1 h. When nitrogen was introduced into the chamber the pressure increased to 101 Pa. Plasma discharge was initiated and high voltage pulses of 20 kV were applied to the sample holder. The current density during the pulse was 15 mA/cm2 (see Ref. [34]), pulse duration 5 ls and the treatment dose 5 · 1015 ion/cm2.

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For the evaporation experiments, vacuum chambers with plasma and ion beam equipments were used at pressures 1 to 103 Pa. These pressures are lower than the vapour pressure for all resin components. The lowest pressure of 103 Pa corresponds to a pressure of residual atmosphere near spaceship in LEO. Therefore the evaporation process corresponds to evaporation in a high vacuum space environment. The FTIR transmission spectra were recorded on Bruker IFS-66 spectrometer with DTGS detector, spectra resolution of 2 cm1. The OPUS software package of Bruker was used for spectra analysis. The FTIR reflection spectra were recorded on a Fourier transform infrared spectrometer (Nikolet Magna 750) with reflection accessory at 8 angle of incident. Spectra were transformed to dielectric constants using the Kramers–Kronig relations. The OMIC software was used for spectra analysis. Analysis of the curing kinetics by FTIR transmittance spectra was carried out on a Bruker IFS-66 FTIR spectrometer. Samples were placed on KBr pellets. A cassette containing the reaction resin mixture was exposed to UV light and FTIR spectra of the samples were recorded periodically. The thickness of the samples after vacuum, plasma and ion beam exposure was determined by mass measurements on a balance and by the absorbance of FTIR spectra lines. Dynamic Mechanical Analysis (DMA) was carried out using the ARES analyser (Rheometric Scientific, Inc., USA). Sample plates of 50 · 10 · 0.3 mm were tested. The rate of heating was 5 C/ min, strain was 0.05 radian with frequency of 1 Hz. Samples were heated and cooled continuously in dry N2 gas flow, from 50 to +150 C. The behaviour of G 0 and G00 modulus and tan d were analysed. Orchestrator software was used for analysis. For kinetics measurements the Blak-Ray UV lamp was placed in the DMA analyser and the samples were irradiated by UV light during measurements. 3. Kinetics of photopolymerisation The photopolymerisation kinetics depends on the concentration of active monomers and UV photoinitiator concentration, as well as temperature and intensity of UV light. The complete description of the photopolymerisation kinetics in liquid polymer mixture can be obtained by equations of chemical process of multicomponent reactionable mixture

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with dependence on elementary rate reactions on UV light intensity. We modelled the photopolymerisation kinetics for the one-dimensional approximation for plane unlimited samples with the assumptions of a homogeneous distribution of initial active molecules and UV photoinitiator molecules, constant temperature, first order reaction products kinetics and absence of convections due to high viscosity of the resin and filler. The photopolymerisation kinetics rate is proportional to square root of UV light intensity [36]: k ¼ k 0  I x1=2

ð1Þ

and I x ¼ I 0 expðeAxÞ;

ð2Þ

where Ix is the intensity of UV light at a distance x from the polymer surface, e is the extinction coefficient for UV light absorption and A is the concentration of the UV adsorbing molecules. The modelling of the UV-curing kinetics was carried out for a first order reaction of polymerisation (Fig. 1): oCðx; tÞ ¼ k  Cðx; tÞ; ot

ð3Þ

where C(x, t) is the concentration of reactant groups. The kinetics curves are presented in values of the reaction coordinate (y) which is determined as y¼

Cðx; tÞ  C 0 C0

ð4Þ

Coordinate of reaction, parts

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

5

10

15

20

25

Depth, μm Fig. 1. Results of UV-curing kinetics simulation. Array shows the time scale with a step size of 5 mins.

for the concentration of reactant and y¼

Cðx; tÞ C0

ð5Þ

for the concentration of the reaction products. For the analysis of the reaction products in whole sample, the concentration of the reaction products is integrated over the sample thickness. The reaction proceeds along a front. The coordinate of reaction has an inhomogeneous distribution over the sample depth. In the case of a high value of UV absorption coefficient, the kinetics front slows down at some depth from the irradiated surface of the sample. The increase of UV light intensity shifts the stopping boundary to a deeper layer within the sample. The kinetics curve of reaction products in all samples with time has an asymptotic character. This reaction is not completed when (i) the sample is too thick, or (ii) the extinction coefficient of UV light is too high, or (iii) the UV intensity is too low. In our experiments, the curing kinetics of a liquid resin irradiated by UV light was analysed by FTIR transmission spectra of the resin on KBr substrate which give the concentration of the reaction products under UV irradiation integrated over the thickness of the sample. With the time of exposure to UV-light irradiation the intensities of the lines at 1658, 1408, 986, 810 and 704 cm1 decrease and the intensities of the lines at 1250, 1119, 1107, 1045, 831 cm1 increase corresponding to the polymerisation reaction (Fig. 2). The kinetic curves have an asymptotic character as was predicted from the modelling above. For example, the optical density of the spectral line at 1250 cm1 is presented in Fig. 3. At the beginning of the polymerisation the change of the optical density is strong, and then the rate of increase reduces. The rate of polymerisation calculated as the first derivative of the optical density with respect to time decreases sharply with time. The theoretical modelling of the photopolymerisation reaction noted above was then applied to our experimental data. The theoretical kinetics curves show excellent agreement with the experimental data. The agreement of experimental and theoretical results is further supported by the analysis of the derivatives of the kinetics curves. The photopolymerisation process was carried out at different UV light intensities. The kinetics curves were observed using the normalised optical density of 1250 cm1 line (Fig. 4). The intensity of the 1185 cm1 line which is stable during the reaction

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1

Absorbance, arb. units

0.8

Time

0.6

0.4

0.2

1500

1000

Wavenumber,

500

cm-1

Fig. 2. FTIR transmission spectra of the resin during curing under UV light of 10.2 mW/cm2 power density. Time scale by array is 0, 29, 64, 99 and 130 min of UV irradiation.

0.05

0.75

0.04

0.71 0.03 0.69 0.02 0.67

dD/dt, 1/min

D(1250), arb. units

0.73

0.01

0.65

Dn(1250), arb. units

0.95

0.85

0.75

0.65

0.55 0

0.63

50

0

10

20

30

40

50

60

Time, min Fig. 3. UV-curing kinetics (cubic) and the rate of curing (triangle) by optical density of the 1250 cm1 line in FTIR spectra of resin. Continuous lines show the results of theoretical modelling.

100

150

Time, min

0

Fig. 4. Kinetics of photopolymerisation in air under UV light of 0.24 (rhombus), 1.3 (triangle), 10.2 (square) mW/cm2 intensity and after vacuum treatment (stars); in plasma discharge (circle); under ion beam (cross). Continuous curves show the modelling results.

4. Evaporation processes of photopolymerisation was used for normalisation of the spectra as an internal standard for different samples. An increase of UV-light intensity causes an increase of experimental kinetics rate corresponding to Eq. (1) and an increase of the last stage of the reaction corresponding to Eq. (2). However, a complete reaction was not achieved.

The resin has a number of different components with different molecular weights and evaporation rates. The evaporation kinetics of a liquid mixture in high vacuum depends on the diffusion and evaporation processes of the separate components. In the case of thin resin samples, the influence of the

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diffusion process can be neglected. Evaporation of liquid resin can decrease a matrix/filler ratio in a composite material and can lead to a decrease in the strength of the composite. The difference between evaporation rates of the active components is critical, because fast evaporation of the active components can destroy the stochiometric ratio of the resin and stop the polymerisation reaction.

For this work, the evaporation process was observed using FTIR transmission spectra of the liquid resin before photopolymerisation (Fig. 5). The intensity of spectral lines in FTIR transmission spectra is described by the Bouger–Lambert– Beer law: I ¼ I 0  eDi ;

ð6Þ

Absorbance, arb. units

1

0.8

Time 0.6

0.4

0.2

1500

1000

Wavenumber, cm

-1

Time

Absorbance, arb. units

0.3

0.2

0.1

3600

3200

2800

Wavenumber, cm-1 Fig. 5. FTIR transmission spectra of uncured resin during evaporation at 1 Pa pressure of residual air. Time scale by array is 0, 10, 22, 40, 80 and 120 min in vacuum.

A. Kondyurin et al. / European Polymer Journal 42 (2006) 2703–2714

where Di = ei Æ ci Æ h is optical density of i-line, ei is the extinction coefficient, ci is the concentration of i-absorbed groups and h is the thickness of sample. In the absence of a chemical reaction the optical density of its absorption lines is representative of the evaporation kinetics of a component in liquid resin. The optical density results of the FTIR transmission spectra with time shows that the evaporation process is complex. The normalised intensities for some of the lines are presented in Fig. 6. The optical densities decrease with time at different rates, corresponding to the different evaporation rates of the components. For example, the intensity of the 1660 cm1 line which corresponds to residual solvent, decreases very rapidly at the beginning of evaporation. The intensities of other lines, such as the 1725, 1635, 1408, 1275, 1062, 985 and 810 cm1 lines, all decrease at a high rate during the whole evaporation process. The intensities of the 3444, 3038, 2967, 2877, 1185 and 1046 cm1 lines decrease more slowly. The intensities of the 2935, 1608 and 1462 cm1 lines decrease significantly at the beginning but do not change after 30 min of evaporation. The intensities of the 1510 and 830 cm1 lines do not change during exposure of the sample to vacuum. The different evaporation rates correspond to changes of relative concentration of the resin components during exposure of the sample to vacuum. This indicates that the resin composition changes during exposure to high vacuum, prior to the commencement of the polymerisation reaction.

1 0. 8

Dn

0. 6 0. 4 0. 2 0 0

30

60 Time, min

90

120

Fig. 6. Normalised optical density of 830 (triangle), 3444 (square), 1062 (rhombus), 1461 (circle) and 1660 (star) cm1 lines in FTIR spectra of liquid resin during evaporation.

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The photopolymerisation kinetics of the resin exposed to vacuum were also measured using FTIR spectra. The liquid resin on KBr window was treated for 2 h in vacuum at 1 Pa, then the photopolymerisation reaction kinetics was carried out under UV light. The kinetics curve determined by spectral data shows the same behaviour of the photopolymerisation reaction as for the initial sample (Fig. 4). This demonstrated that these vacuum conditions do not affect the photopolymerisation reaction of the resin significantly. 5. Photopolymerisation in plasma and ion beam Space irradiations have a great influence on polymer materials. A complete simulation of the free space environment is not possible in a laboratory because of the complexity of space irradiations by particle contamination, density of fluxes and energies. The fundamental effects of a space environment on the structure and mechanical properties of polymers can be simulated using plasma techniques. In our experiments, the plasma and ion beam treatments were used for simulation of a free space radiation environment’s effect on the liquid uncured resin. The liquid resin was put onto a KBr window, which was then placed in the plasma chamber. The surface of the liquid resin was exposed to the plasma discharge and ion beam. After plasma treatment FTIR spectrum of the resin was recorded and the plasma treatment repeated. The FTIR transmission spectra of the resin exposed to plasma discharge can show the evaporation, polymerisation and etching processes (Fig. 7). The optical densities of all the lines decrease with time during plasma treatment. The intensities of the 1730, 1608, 1510, 1462, 1408, 1250 and 1185 cm1 lines all decrease at the same rate indicating that the etching of the different components of the resin occurs at the same rate. These changes are different from those which occur during exposure to vacuum, where the intensities of different lines change at different rates. The normalised optical densities of the spectral lines were used for analysis of the polymerisation process. The normalised intensities of 1250, 1119, 1107, 1045 and 831 cm1 lines increase and the normalised intensities of 1658, 1408, 986, 810 and 704 cm1 lines decrease corresponding to the polymerisation reaction as it was observed under UV irradiation. In addition, the chemical degradation

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Time

Absorbance, arb. units

0.4

0.2

0

1500

1000

500

-1

Wavenumber, cm

Fig. 7. FTIR spectra of the resin during plasma treatment. Time scale by array is 0, 15, 30, 45, 60, 75, 97, 112 and 127 min in plasma discharge.

of the resin during plasma treatment was also observed. The normalised intensities of the 3440, 1385 and 1363 cm1 lines increase with time of plasma treatment with a new line appearing at 785 cm1. These spectra data shows that the structure changes of the polymerising resin in plasma discharge are complex. The spectra changes of the resin after ion beam treatment are quite similar to the spectra changes after plasma treatment. In comparison with the spectra after plasma treatment, the spectra of resin after ion beam treatment have lower intensities of the 1384 and 1363 cm1 lines. These lower intensity lines correspond to a lower destruction process in the resin macromolecules at ion beam treatment. The intensity of the hydroxyl group line at 3440 cm1 is lower than in initial sample reflecting a decrease in the concentration of hydroxyl groups. On the other hand, the normalised intensities of 1250, 1119, 1107, 1045 and 831 cm1 lines interpreted as results of photopolymerisation process are higher than for plasma treated samples. This shows that the polymerisation reaction reaches more complete stage after ion beam treatment than after plasma treatment. After these experiments it becomes clear that the space factors simulated by plasma and ion beam initiate the photopolymerisation reaction without UV irradiation from UV lamps. The analysis of the opti-

cal density of the 1250 cm1 line shows that the polymerisation process in plasma discharge and ion beam is more intensive and the reaction of the photopolymerisation reaches higher stages than under UV light (Fig. 4). The theoretical approximation of the photopolymerisation kinetics under plasma and ion beam shows that the photopolymerisation reaction behaviour is similar to that of UV light. However, UV light intensity in plasma discharge and ion beam is significantly lower than that used in UV lamps. In addition, the increasing effect of the photopolymerisation reaction in plasma and ion beam is caused by the additional short wave number UV light, high energy electron and ion fluxes. These irradiation components are able to initiate the photopolymerisation reaction in the resin by the same way of an excitation of the photoinitiator molecule. Such high energetic irradiations shift the profile of the photopolymerisation reaction deeper creating a thick and complete polymerised layer of the resin. In a free space environment these components of irradiation are present together with additional X-ray and c-irradiations which can initiate the photopolymerisation reaction. This indicates that the photopolymerisation reaction can occur at a fast rate in real space environment without the necessity of exposure using UV lamps. The surfaces of the initial resin and resin cured after vacuum exposure are smooth. The surfaces of

A. Kondyurin et al. / European Polymer Journal 42 (2006) 2703–2714

Fig. 8. Microphoto of glass composite with resin cured under ion beam.

the resin cured in plasma discharge and ion beam are significantly rough. The roughness is not homogenous on the sample surface. Some parts of the surface are similar to a frozen flow (Fig. 8). Other parts of the surface contain a wave structure similar to wrinkling of the thin surface layer under high internal stresses. Other parts of the surface are sufficiently smooth similar to the resin cured under vacuum. And there are parts of the surface which correspond to the appearance of solid polymers etched under plasma discharge. The same complex structure of the surface is observed in photopolymerised composite with glass fibres in plasma and ion beam. Similar structure of the surface was also observed for epoxy resin composition cured by thermal initiation reaction under plasma and ion beam [34]. This complex structure is due to the flowing processes in the resin during polymerisation under plasma and ion beam treatment, when the resin has sufficiently low viscosity. During polymerisation reaction, the viscosity increases and the flowing structure freezes, creating a mixture of different layers of the resin on the surface. This is the reason for the acceleration of the photopolymerisation reaction. 6. Photopolymerisation kinetics at the temperature variations during space orbital flight The structure changes caused by photopolymerisation processes of the composite under vacuum,

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plasma and ion beam have an influence on the mechanical properties of the composite. The mechanical properties evolution during the kinetics of photopolymerisation is important for an inflatable structure, because the rigidization process is carried out in mechanically loaded material and the mechanical loads change due to temperature changes during flight. The temperature variation of the inflatable constructions depends on the flight regime and sun irradiation. In real space flight on the low Earth orbit, the photopolymerisation process of the composite material occurs over a wide temperature range. In this study, we have considered the low Earth orbit flight (300–400 km height) of an inflatable construction (Fig. 9). This orbit is typical of manned flights used in the ISS and Photon satellite missions. The temperature of the construction depends on the phase of the flight. At point A, the construction becomes irradiated by sun light and the temperature of the construction increases with time. At point B the construction is shadowed by Earth and the temperature decreases with time. The kinetics of UV-curing process was measured by the DMA method in situ. Mechanical properties of the composite samples consisting of glass textile and liquid resin were measured under UV light irradiation. The real G 0 and imaginary G00 components of modulus with tangent of mechanical loss angle of the composite were analysed during the photopolymerisation process. The real component of elastic modulus G 0 increases with time from the beginning of UV light irradiation (Fig. 10). This is caused by the increase of the resin viscosity during photopolymerisation

A

Sun light

Shadow side

Earth

Flight direction

B

Fig. 9. Low Earth orbit flight of the inflatable construction during UV-curing. At point A the construction is irradiated by sun light, at point B the construction comes into the Earth’s shadow. Drawing not to scale.

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G'

7.E+09

0.12

6.E+09

0.8

0.1 0.08

4.E+09

Tan (δ)

3.E+09

0.06

2.E+09

0.04

0.6

G' n

5.E+09

Tan (δ)

G' and G'', Pa

1.0

0.14

0.4

G'' x5 Start of UV irradiation

1.E+09 2.E+07 0

50

100

150

200

0.02

0.2

0 250

0.0

Time, sec

0

100

Fig. 10. DMA kinetics of UV-curing of the glass fibre composite.

reaction. The increase of G component of modulus occurs at a low rate, when compared to the chemical groups transformation as observed by FTIR spectral data. The kinetics curve of G 0 modulus has an asymptotic character and reflects a formation of polymer network in the resin during photopolymerisation reaction. The imaginary G00 modulus and tan d increase quickly at the beginning of the reaction. The time dependence of the G00 component is similar to the photopolymerisation reaction kinetics as determined by spectral data. The imaginary G00 modulus reflects viscous mechanical losses in material and an increase of this modulus corresponds to a decrease of the macromolecular mobility. The photopolymerisation kinetics were analysed in a temperature range similar the real temperature variations during the low Earth orbit flight. The composite plate with liquid resin was irradiated by UV light at different temperatures and the mechanical parameters measured during curing kinetics. The normalised elastic modulus curves at different temperatures are presented in Fig. 11. The elastic modulus shows a strong dependence with respect to the temperature of the sample. The rate of increase of the modulus during reaction increases with temperature elevation. Temperature simulation of the orbital space flight was applied for photopolymerisation kinetics of the composite. The low Earth orbit flight regime was used for temperature behaviour simulation. The temperature was varied from 10 to +80 C and back at a rate of 2 C per minute. This corresponds to a low orbital flight of a 90 min cycle. We have considered that the inflatable process will occur on the shadow side of the Earth to exclude sun irradiation during inflation process. In

300

400

Fig. 11. Normalised elastic modulus of the glass fibre composite during photopolymerisation at temperatures of 80 C (rhombus), 50 C (square), 15 C (circle), 0 C (cross) and 10 C (triangle).

this case, the curing process starts at a low temperature. In our experiments the UV irradiation is started at lowest temperature of 10 C which corresponds to a position of the inflated construction at point A in Fig. 9. The behaviour of G 0 real component of elastic modulus with time of the flight is presented in Fig. 12. During flight from position A to B the temperature increases, the photopolymerisation reaction occurs, but G 0 modulus does not change much because of simultaneous heating and curing reaction. At point B the temperature reaches highest value and starts to decrease. The modulus increases due to cooling of the composite.

3.0E+10

2.5E+10 2.0E+10

G', Pa

0

200

Time, sec

2nd turn 1.5E+10

3rd turn 1.0E+10

5.0E+09 0.0E+00 -20

1st turn

0

20

40

60

80

Temperature, C Fig. 12. Elastic modulus kinetics of the composite during simulated low Earth orbit flight.

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At next cycles the modulus follows the temperature variations as for cured composite materials. The complete photopolymerisation kinetics was carried out during one turn of construction around Earth. 7. Conclusions The investigation of the curing process of the composite material under simulated free space environment shows that the photopolymerisation reaction can occur in free space environment. The photopolymerisation kinetics is described by the model of frontal reaction with slowing down in the internal layer of the resin with time. The evaporation process occurs in high vacuum similar to real conditions in free space environment. The composition of the resin is observed to change at different rates due to evaporation of the different components of resin. The photopolymerisation reactions have been compared using plasma and ion beam processes. We observed that in these two processes, the reaction of photopolymerisation becomes more complicated due to the degradation of the macromolecules. The rate and depth of the photopolymerisation reaction is higher than for UV lamps irradiation. Together with high energetic space irradiations all these space factors have been shown to initiate the photopolymerisation reaction without the necessity of exposure using UV lamps. Therefore, the photopolymerisation reaction of inflated construction in space will start immediately after inflation of the construction, even before the operation of UV lamps. The mechanical properties behaviour of the composite based on the photopolymerising resin matrix were observed in experiments of low Earth orbit flight simulations. It was found that the photopolymerisation reaction changes the elastic modulus of composite in dependence on the temperature and phases of orbital flight. The most changes of the elastic modulus were observed during the first turn of the construction around Earth. Due to high effectiveness of the photopolymerisation reaction in free space environment, the photopolymerisation technology can be used for large-size inflatable constructions in Earth orbit, in far space and on space bodies as for deployed antennas, solar sail stringers, solar shield stringers, frames for large-size space stations, frames for Moon, Mars, asteroids bases and frames for space plant on Earth orbit and other celestial bodies.

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Acknowledgements The investigations were supported by Alexander von Humboldt Foundation, European Space Agency, ESTEC (contract 17083/03/NL/Sfe ‘‘Space Environmental Effects on the Polymerisation of Composite Structures’’), scholarship of Institute of Polymer Research and Russian Foundation of Basic Research (grant 05-08-18277). The authors express gratitude to Prof. Marcela Bilek and Dr. Bee Kwan Gan for proof-reading the manuscript, friendly advises and useful discussions. References [1] Wilson A. A history of balloon satellites. J Br Interplanetary Soc 1981;34:10–22. [2] Cadogan DP, Scarborough SE. Rigidizable materials for use in Gossamer Space Inflatable structures. AIAA 2001:1417. [3] Cadogan D, Stein J, Grahne M. Inflatable composite habitat structures for Lunar and Mars exploration. In: 49th International astronautical congress 28 September–2 October 1998, Melbourne, Australia, IAA-98-IAA.13.2.04. [4] Salama M, Lou M, Fang H. Deployment of inflatable space structures: a review of recent developments. AIAA 2000:1730. [5] Sasakawa International Center for Space Architecture, SICSA outreach, Special Design Project Issue, vol. 1(7), 1998. [6] Allred R, Hoyt AE, McElroy PM, Scarborozgh S, Cadogan DP. UV rigidizable carbon-reinforced isogrid inflatable booms. AIAA 2002:1202. [7] Bar-Cohen Y. Transition of EAP material from novelty to practical applications – are we there yet? In: SPIE’s 8th annual international symposium on smart structures and materials, Newport, USA, 5–8 March 2001, Paper No. 432902. [8] Cadogan DP, Scarborough SE, Lin JK, Sapna GH. Shape memory composite development for use in Gossamer space inflatable structures. AIAA 2002:1372. [9] Darooka DK, Jensen DW. Advanced space structure concepts and their development. AIAA 2001:1257. [10] Darooka DK, Scarborough SE, Cadogan DP. An evaluation of inflatable truss frame for space applications. AIAA 2001:1614. [11] Grossman G, Williams G. Inflatable concentrators for solar propulsion and dynamic space power. J Solar Energy Eng 1990;112:299. [12] Veldman SL, Vermeeren CAJR. Inflatable structures in aerospace engineering – an overview. In: Proceedings of the European conference on spacecraft structures, materials and mechanical testing, Noordwijk, the Netherlands 29 November–1 December 2000 and ESA SP-468, March 2001. [13] Cadogan D, Grahne M, Mikulas M. Inflatable space structures: a new paradigm for space structure design. In: 49th International astronautical congress, Melbourne, Australia, 28 September–2 October 1998, IAF-98-I.1.02. [14] Cadogan DP, Lin JK, Grahne MS. Inflatable solar array technology. AIAA 1999:1075.

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