Wednesday, August 07, 2013

More on Using Plastics for Shielding from Cosmic Radiation, Part II

Back in June, I wrote about using plastics as radiation shielding on spacecraft, and even had a followup on correspondence with the author regarding the degradation of the polymer over time. I promised to get myself to the library [*] sometime to look up tabulated data on the matter that I knew existed in the Polymer Handbook, a book located in the reference section of the library.

The Polymer Handbook has a chapter in Section II about the Radiation Chemical Yields of polymers exposed to radiation. What I like the most about the chapter is that it has columns for both the crosslinking yields and scissioning yields, often for the same polymer. That's right; when exposed to radiation, polymers do not just crosslink or just get degraded, but both reactions occur. It's just that one reaction tends to dominate, a concept that is lost on too many people (as we will see shortly).

As was previously reported, the A-150 Tissue Equivalent Plastic is a blend of polyethylene and nylon 66 with some fillers. The tabulated data showed that the crosslinking reaction for polyethylene dominates the scissioning reaction, but sadly, the book is silent on nylon, or at least aliphatic nylon. I was able to locate a reference on the radiation of nylon which states: "Polyamides crosslink and lose crystallinity upon irradiation causing a slow increase in tensile strength but a much more rapid drop in impact strength. The latter characteristic falls to half its initial value after a dose of about 20 Mrad." Notice how that the author falls for the false choice of crosslink/scission?

So it appears that the A-150 will undergo mostly crosslinking reactions in space, resulting in a increasingly brittle material. That is still preferable to the other option of chain scissioning which would only weaken the material over time.

[*] What's a library you ask? Well, it's this building that has shelves filled with these old information and storage media called "books". Books were made by chopping down trees and using the cellulose in them to make flat sheets of material called paper. The paper could be marked with a colored liquid called ink. If the ink formed patterns similar to those generated by the pixels on your IPhone or IPad, people could read them. The only problem is that the marks were more or less permanent and so each book had a fixed set of information in it.

Stop laughing! I'm not making this up. This is how people use to get information.

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