Plastic Degradation Due to UV Light

From: Temple of Primus
Subject: Reply

On Wed, 29 Nov 2000, Fred's Workshop wrote:

...because you are the 'man in the know' on chemical compounds. :-)

You flatter me, Fred.-- I was in L.A. for a chemical engineering conference 2 weeks ago, then had to leave sat after thanksgiving for another materials conference in Boston. I just got back last night.

We've been told that he decay rate of plastic is about 20 years. Does this mean that some of my oldest stuff is even now turning to dust, or engaging in a chemical meltdown? Like those Barbie Dolls of years past?

Yes and no. The stories about the barbies is no doubt true. Without knowing what polymer additives were used by Hasbro, I think it's a safe bet to say it was polyisoprene or maybe styrene butadiene rubber, not the phthalates used in barbie. So they're not made of the same plastic.

The upshot is that all plastics degrade. This is done in 2 ways: temperature and time. Time you can't help. Temperature you can. Basically, the lower you keep it (within reasonable bounds -- freezing it would be bad also -- it would make it brittle), the better. The phenomenon is based on time-temperature superposition (if you want to read more about it in a basic polymer textbook). It means that time and temperature have the same effect in degradation. Agitation of the molecular bonds of the plastic are what encourage it to degrade. So in a strict sense, yes someday they will degrade. but it's highly unlikeley that will be a problem in our lifetimes if toys are not left out in the sun, say, for a whole afternoon. I think having them in cabinets in a climate-controlled room is about as safe as one can make them! The famous jetfire discoloration is a result of energetic photons from light degrading polymer chains. Since it takes a LONG time to make it actually crumble macroscopocially, you see this as a discoloration. Essentially, a thin film is being caked away.

The reason light is bad is because it's one medium for transferring energy. UV light is more energetic than normal light and so is worse -- just like getting a tan.

I'm still playing catchup at work, so you may have to wait for the sequel! Actually, I've been meaning for years to write more on this for my technical FAQ at my website. Alas, the rigors of school. Any questions, as always, feel free to ask.

(See Also: Plastics Used in Making Testshots and Other TFs)