Thursday, 17 May 2012

Curing at space temperatures

Well, go to next problem. Temperature.
The temperature in space is tricky. Usual imagination, what we have on Earth, does not work there. Imagine: there are no walls, furniture, ground, neighbours, friends and wind (air) surrounded your body. Temperature depends on the irradiation from the Sun and on internal sources. All bodies irradiate following Stefan–Boltzmann law. 
If no irradiations from neighbours, your temperature goes to absolute zero, well, not zero, because of space irradiations. But anyway, it comes to very low temperature; the space flight measurements give temperatures about -150 C. If you are irradiated by the Sun, your sun-side will be heated up in dependence on reflectivity of your surface, thermocapacity, thermoconductivity and geometry of your body. Very quickly you start to feel, that one side of you is burning (can be +150 C and more), while the other side is still frozen. You will want to turn. If you turn quickly enough, your temperature will be more or less steady. So, the rotation of the body is very important.
Now let’s remember, how the chemical reaction depends on temperature. At first approximation, the rate of reaction follows Arrhenius law: the rate increases with temperature. Usually, the curing system is selected to be non-reacting at storage temperatures so, that uncured material can be kept in container at transportation without the reaction. Therefore, the temperature at curing should be higher than on Earth and at transportation.
There are some ways to get it. First of all, rotation of the construction side by side to the Sun should be so, that each part of the construction will be heated enough to be cured completely. It does not need massive efforts, because no friction there, and if the construction is accelerated it will rotate forever. You have to be smart enough to calculate the rotation speed and direction. It can be calculated, measured, compared with experiments on orbit. The regime of rotation can be optimised to get complete curing in all sides and parts of the construction.
But what can you do on the Moon or on an asteroid? You cannot rotate the Moon of asteroid as we want. If you are settled down on equator of the Moon, you can expect heating enough with the turning of the Moon. But if your construction has to be placed on polar, what is more likely because of found water there, there is no way to get a heat enough. You have to heat it with internal sources, for example, with internal electrical heaters. And you have to be ready to spend a lot of energy during curing reaction. It is not a huge amount of energy: ISS astronauts spend a comparable amount of energy to support life there. The construction can be heated partially: sector by sector, that can decrease an amount of power, you have to apply.
Another way is to use photocuring reaction. There are compositions that can be cured under UV light. That’s nice way if you need to cure quickly, on command after storing long time in the container. In such case, the curing reaction is not so sensitive to the storage temperature, while the rate of curing anyway depends on temperature. Such photocuring systems can be suitable for repairmen set. But the photocured materials have usually lower mechanical strength, lower radiation stability, shorter life-time and narrower diapason of exploitation conditions, than thermocured materials. These two kinds of materials (photocured and thermocured) are specialised for different constructions. You can choose one of them for particular construction and exploitation conditions.
However, there is a serious problem with the temperature in space: thermostresses. This problem needs attention. Usually, on Earth the prepreg (uncured material) is placed into curing oven, heated slowly with optimised rate of the temperature increase, cured at uniformly distributed temperature, and cooled slowly. The heating/cooling process is optimised to avoid the thermostresses in the construction. In space, when the construction is irradiated from one side, the Sun irradiation creates a temperature gradient. The curing reaction follows to the temperature gradient in the construction. Therefore, the different parts of the construction will be cured at different temperature and will keep memory of the temperature gradient. When the cured construction changes an orientation, the temperature gradient changes and it generates the stresses. As higher temperature gradient is at curing, as higher stresses appear. The stresses deform your construction and decrease the mechanical strength of the construction.
Because no one large curing oven with temperature stabilisation is installed in Earth orbit, where you can put your construction for precise curing, the temperature regime of the construction should be precisely calculated and the flight regime should be optimised to get completely cured material of the construction without significant stresses.

Monday, 14 May 2012

Curing in free space plasma

Free space plasma, deadly space irradiations, burning Sun light. Who can live there? What material can survive there? For how long? – Hopeless questions. That’s enemy environment for all Earth-born stuffs.
All materials, including polymers, degrade in space environment under high energy cosmic rays, Sun wind, atomic oxygen of residual Earth atmosphere (if Low Earth Orbit). Since first space flights, engineers worry about degradation of the materials used for space ships, satellites, stations. A number of experiments have been done, when different kinds of materials were exposed on Low Earth Orbits. Then the materials were delivered on Earth, to laboratories for an investigation. The structure changes in all polymer materials have been observed, described, calculated and simulated in laboratory experiments.
First of all, this is an effect of etching. The materials disappear with time: layer-by-layer. You can find a rate of etching in literature for different kinds of materials. There are handbooks, database, standards and recommendations how to choose a right material based on mission, orientation, lifetime and functionality of materials in particular space construction.
At second, the materials become brittle, cracked, and finally broken under space conditions. The molecular structure changes significantly: polymers become crosslinked, depolymerised and oxidised in dependence of kind of polymer. All these effects in polymers can be observed in laboratory under plasma and high energy particles. The chemistry of these processes is based on generation of free radicals, when a high energy particle hits a macromolecule and forms free radicals. The free radicals are very active and start to react with neighbour macromolecules. These chemical reactions transform the initial macromolecules dramatically.
The same radiation effects are observed in macromolecules when uncured polymer with liquid matrix is exposed in UV light, g-irradiation, X-ray beam, plasma and ion beam. 
At first, the etching rate is higher. The uncured polymer degrades quicker than the hard polymer. We measured it. But the difference is only 2 times. Is it significant? Yes, for first two-three hours. But then the polymer becomes hard and stays 15-20 years. Therefore, the contribution of high etching rate, when the polymer was liquid, is neglectable in comparison of low etching rate at the rest of life. 
At second, the radiation damaging of the macromolecules is the same. The generated free radicals in matrix can cause two kinds of reactions: crosslinking and depolymerisation. If right composition is selected, the crosslinking reactions proceed and the polymer matrix becomes hard. The same effect as in curing reaction, but without any hardener! Therefore, the free space environment can play a role of additional initiator of the crosslinking reaction. 
"The space makes polymer hard", as the journalist wrote about our investigations. That’s true, in the case of uncured composite the enemy space environment helps us to get durable material. Let’s use this help smartly. 

Tuesday, 8 May 2012

About curing in vacuum

Well, let’s consider the problems of polymerisation in space: first of all is vacuum.
Low pressure of free space environment is a problem for all materials, constructions and human during a space flight. This is unusual in comparison what we have on the “bottom” of our air “ocean”.
At first, the residual gases can inflate the shell of the construction shortly after launch, when the container with folded shell is lifted to space. The inflation pressure is very low, if outer pressure becomes neglectable. This was a reason of some failed space flight missions when large shell was inflated in Earth orbit, but the uncontrolled inflation broke the shell. The inflating pressure is close to vapour pressure of cured (hard) polymer materials, which always contain some dissolved gases, low molecular fractions and residual solvents. The presence of residual gases is so dangerous, that the polymer shell can inflate spontaneously after opening of the container.
What will be, if a liquid resin with high vapour pressure will be placed into the hermetic shell? Explosion. This is why most projects on inflation space construction with uncured material inside are not realized. No one of material experts in space agencies agrees to sign permission, that the shell with liquid resin inside will be deployed under control.
Can we manage it? Yes, we can.
The prepreg with liquid resin should be placed on external side of the inflating shell. In such case, the evaporation of liquid resin will be into space. The inflation process of the shell remains dangerous due to shell vapour pressure, but with special ventilation we can decrease the pressure caused by evaporation of low molecular components from the shell material. And the high vapour pressure of the uncured resin will be not important for the inflation.
You can ask me:
- wait a minute, it means, that the uncured liquid resin will be placed directly into space?
- Yes!
- But the resin components will evaporate and disappear with time. Nothing will remain for curing!
- Yes, if a composition of the resin is wrong. Some people from ESA tried it, failed and said: “curing in vacuum is impossible”. However, if the composition is right, the evaporation of components is not a problem.
How can we select a right composition?
Look, if you put a glass of water in vacuum chamber and pump it, after some time you will see, the water evaporated completely. If you put a glass of ethylene diamine (the hardener for epoxy resin) into vacuum chamber and pump it, the ethylene diamine will evaporate too. However, if you look at the door of vacuum chamber, you can see a rubber O-ring. It is used for hermetisation of the vacuum chamber door to prevent air coming. This O-rig does not disappear after long pumping at extremely low pressure and temperature. You see, there are soft substances that can survive in vacuum. Actually, all materials evaporate in vacuum including metals, the question is: how fast? We must select substances suitable for the curing (active) and survival in low pressure (slow evaporation).
To estimate the dangerous of evaporation, we have to consider a curing reaction of the polymer matrix together with evaporation. In literature you can find plenty information about kind of curing reaction, some of active compositions are certified by space agencies to be used in space for construction materials. All of these materials consist of minimum two components: resin and hardener. The reaction of polycondensation is mostly used for curing of such kind of materials. It means, that the ratio of resin and hardener is usually optimised to get durable composite after curing. If one component is lost, the composition remains uncured and the material lost mechanical characteristics.
Therefore, the right composition should provide low evaporation rate for both active components: hardener and resin, and the rates of evaporation should be similar for all active components.
The second problem is cavitation. When uncured liquid composition contains a lot of low molecular fractions (it does not matter, if they are active or not) and these fractions evaporate fast, the composition becomes bubbled in vacuum. These low molecular fractions evaporate too fast and the vapours are collected into bubbles. If the composition becomes harder with time, the bubbles cannot move to the surface, stop and form foam. You can see it, if you buy liquid polyurethane in nearest tool shop and make polyurethane foam. Similar foam was observed in NASA space experiment during space flight and they said: “curing in vacuum is impossible”.
Therefore, the right composition should not contain low molecular fractions which can make bubbles and foam in vacuum.
If the composition does not break the curing reaction and does not give a foam in vacuum, it can be cured in space. That’s just right selection based on knowledge of the evaporation rates, composition components, curing kinetics and some experience. We have found and tested some compositions up to 10^-5 Pa pressure. They are not expensive and not rare. Some of them in cured form are certified for space constructions can be used now.
If pressure becomes lower than the vapour pressure of the components (10-100 Pa for liquid epoxy resins, for example) and the evaporation has been started, a following decrease of the pressure does not play a role. For example, if the pressure in Low Earth Orbit is 10^-5-10^-7 Pa (while the pressure near spaceship depends on sun irradiation, how long is the ship in the orbit, material of the ship walls and so on and it is usually higher than the pressure far from the ship), the evaporation will have similar effect on the curing material as in deep space, when the pressure can be 10^-11 or lower (if no one has been there and did not put his gases, I mean evaporation). Therefore, the compositions tested in Earth orbit can be used on Moon, on asteroids, in Jupiter’s orbit and in another galaxy.
So, a curing of liquid composition in vacuum is not a problem, while some official referee of my project in Europe said: “that’s impossible!” and rejected the project.

Wednesday, 2 May 2012

Project:”Large-size antenna dish, shield and frame of space station by the way of polymerization of composite material on Earth orbit in free space environment

The main goal of the project is the development of the polymerization processes of polymer composite materials in free space environment and the creation the technology for large-size constructions on Earth orbit.

The size and mass of modern space constructions (antenna, space satellite, space station or space base) sent to the Earth orbit are limited by possibility of a launch vehicle. The large-size construction can be created by the use of the technology of the polymerization of fibers-filled composites and a reactionable matrix applied in free space or on the other space body when the space construction will be working during a long period of time. For example, the fabric impregnated with a long-life matrix (prepreg) is prepared in terrestrial conditions and, after folding, can be shipped in a container to orbit and kept folded on board the station. In due time the prepreg is carried out into free space and unfolded by inflating. Then a reaction of matrix polymerization initiates. After that, the artificial frame can be fitted out with the apparatus or used for any applications.

In this case, there is no limitation for size and form of the space construction, there is no necessity for some launch vehicles for the creation of high-size space construction.

However, conditions of free space have a destructive influence on polymer materials and especially for uncured polymer matrix of composite. In the free space the material is treated by high vacuum, sharp temperature changes, plasma of free space formed by space rays, sun irradiation and atomic oxygen (on low Earth orbit), micrometeorite fluency, electric charging and microgravitation. Our preliminary studies of polymerization process in high vacuum, space plasma and temperature variations showed that the polymerization process is available in free space under space factors and the composite cured in simulated free space environment has satisfied mechanical properties.

The present project includes:
- Investigation of the polymerization process and structure of selected composite material in simulated space environment;
- Test of polymerization of selected composite material during space flight;
- Development of large-size mirrors, antennas (some km diameter) and frame of space construction (for example, cylinder of 100 length and 10 m diameter) on Earth orbit by way of curing of polymer composite materials in free space.

Dr. Alexey Kondyurin