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.

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