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. 

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