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Name: Brian M.
Status: Student
Age: 14
Location: N/A
Country: N/A
Date: March 2004


Question:
Can UV penetrate through plastic? If so, what types? Does it depend on the thickness? I am doing an experiment with UV and i just want to make sure that it can go through, and if so, what the strength would be if the penetration power is affected by the thickness, in Percent power per measurement in thickness.



Replies:
Dear Hyper-

It really depends. Get yourself a lot of clear plastics to try, because some will go through, some won't at all, some will depend on thickness, and some won't. Beware of scattering and purpose-added UV-blocking dyes. Find out the wavelength dependence of your UV meter and your UV source (the sun?).

Most plastics that are not clear will block most or all of the UV. Even clear colored dyes usually absorb some UV. And anything that is flat-colored or opaque doesn't just absorb light, it scatters light. Try looking at the shadow of a piece of wax paper. It absorbs almost nothing, but most of the light gets bent or reflected somewhere else, so little of the light goes straight through, so it has a fine shadow, especially if it is far from the ground.

To be able to test thin sheets of these things that scatter, you may want to tune up your measurement methods to allow a small amount of scattering. Largely this means putting the plastic sheet right up close against the entrance hole of your UV meter. If the percentage transmission decreases when the plastic backs away from the meter, then it's scattering loss not absorption loss. Having a pre-scattered source of UV light is another way to tolerate scattering. The sun in a clear sky gives mostly direct, unscattered UV light. The blue sky all around contributes some scattered UV light. A cloudy but hot day, the kind when they warn you about sunburn, gives only scattered UV. So wax paper will give a different transmission percentage on a cloudy day than a clear sunny day, especially if the paper is 1 inch from your UV meter instead of right on it.

I know the formula for absorption loss vs. thickness. But I think the formula for scattering loss vs. thickness is different, when the scattering gets high, and I do not know how it goes. Maybe it would be a good goal to measure a percentage-transmission curve for that and try to find a formula that fits it.

Home Depot sells plastic sheets of Lexan (poly-carbonate) which are printed with claims of blocking UV. Try this stuff. You might find it blocks only 90%, depending on your UV meter. I do not think it depends on thickness, though. I think they take a clear plastic that transmits most UV, then they soak into one face a "UV dye" that absorbs only UV, just like the PABA or other active chemical in sun-screen lotions. Even says on the cover sheet: "this side towards the sun". This is the side with the UV-blocking dye, less than 0.1mm deep. A lot of plastic products that are supposed to live outdoors have UV-absorbing dye in them, otherwise the UV light sailing right through all day occasionally cuts a long chain molecule and the plastic eventually browns and cracks. Sometimes they call this "UV inhibited" instead of "UV dyed" or "UV blocked".

Most clear plastics transmit UV if they are chemically pure. The chemically pure plastics that block UV usually block just a little blue light as well, and therefore have a faint yellow or brown tint. I doubt you will find many of those around.

But I could list specific plastics.

Pure Plexiglass ("PMMA", poly-methyl-meth-acrylate) transmits most of the UV that will give you a suntan. Clear poly-styrene plastic is chemically simple, just C's and H's (Carbon and Hydrogen), and no big electron clouds. So it transmits UV better.

Poly-ethylene is even simpler, and will transmit even farther into the UV. But it always has scattering, always looks cloudy or milky (translucent).

DuPont's Teflon (TM) has only Carbon and Fluorine atoms, and transmits so far into the UV that scientists have difficulty getting UV lasers to cut it. But you have seen that it is really white ("PTFE" type), not just cloudy like poly-ethylene, and this kind of

DuPont's Teflon (TM) has very strong scattering. If it is too white to see through, you will get a low percentage transmission in the UV too. Then there are clearer types ("PFA" type) which are only a little cloudy. They transmit well too. Good thing is, almost never does someone bother to put UV-blocking dye in Teflons.

Go ahead and throw wax-paper into your study. The wax is chemically similar to the poly-ethylene above, plus there is paper, cellulose, (-HCOH-) chains, which also transmit most UV. It has some scattering, of course. Any clear colored plastic, including clear fluorescent orange or green, is a good thing to try. Many dyes that block visible colors will miss the UV a little bit, so the plastic would then transmit a moderate percentage of UV and depend on thickness. I'm not sure whether reddish, greenish, or bluish dyes are more like to miss the UV.

Drug-store mineral oil is a good thing to put between multiple sheets of plastic, so the surface-reflections do not add up. When you stack up 10 layers of thin clear sheet, sometimes it looks really shiny, silvery. One sheet was just clear! Oil in between will prevent multi-surface reflections from building up. Mineral oil will not dissolve your UV meter, or hardly any plastics. The oil is C's and H's just like polyethylene, but it is liquid because the molecules are shorter and/or smaller. So it really transmits UV. Mineral oil usually contains a fraction of a percent of an additive for "anti-oxidant" purposes. This anti-oxidant probably also absorbs UV, but maybe not enough of it to completely block your UV. Percent transmission versus mineral-oil depth is a nice little project all by itself, and if the resulting curve shows that it only absorbs 20% in 1 cm, then you have proved that it transmits >98% when you use less than 0.1 cm to wet a bunch of plastic sheets together.

I guess, from your language, that you have a UV photometer of some kind. Really good. But UV is not just one color, it is a range of colors that some other imaginary species can see. So try to find out what wavelengths your UV meter "sees". Include it in your report, so people can know well what you tested. Remember that there is visible blue on one side of it and farther UV on the other side of it. And sometimes it might be measuring the sum of two different colors, with two different absorption rates in the plastic.

Jim Swenson


The transparency of a plastic to UV light is thickness dependent, very dependent. At a given wavelength. Roughly: log (transparency) ~ thickness, so in general "thin is good". It is also dependent upon the wavelength of the UV light -- the longer the wavelength the more transparent the plastic is. The plastic must be "clear". Any pigmentation will cause the light to scatter. A way of testing the transparency is to place the plastic sheet between the UV light source and a piece of high quality white paper. The paper will fluoresce a bright blue if there is no plastic present. If the bright blue disappears when you put the sheet of plastic in between, the plastic is absorbing the UV light. A thin sheet of clear Plexiglas would probably be your best bet, but it does depend upon what has been formulated into the sheet.

Vince Calder



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