Uncovering the Mystery of UV Degradation in Polymers

[Beasticly]

Hey guys, just trying to get my head around this one..

Why do UV rays effect polymer chains so much? I understand that they weaken the bonds within the chains to create free radicals which then go on to react with other things which eventually makes the whole chain degrade as a whole, but why are UV rays in particular so harmful?

Don't they have less energy/photon than visible light, which would make polymers mores susceptible to degration from visible light? Or is it something like there's more UV light rays than visible light rays?

I'm confused!

Thanks!

Also, I wasn't sure to put this in the physics section or here so feel free to move it if I've made a boo boo. :)

 

[Mike H]

Find a reference which breaks down the electromagnetic spectrum into the categories by wavelength/frequency. Use the E = hv (or its wavelength equivalent) relation to determine the energy of a photon in the infrared part of the spectrum, the visible part, the ultraviolet part, and the x-ray part. Which is the most energetic? What region of the EM spectrum is used to probe molecular vibrations? What region of the EM spectrum is used to probe electronic transitions? Do this first, and I think most of your confusion will be cleared up.

Good luck!

 

[Redbelly98]

Don't they have less energy/photon than visible light, which would make polymers mores susceptible to degration from visible light?

Nope, shorter wavelength = more energy per photon. So u.v. photons are more energetic than visible.

 

[GCT]

Hey guys, just trying to get my head around this one..

Why do UV rays effect polymer chains so much? I understand that they weaken the bonds within the chains to create free radicals which then go on to react with other things which eventually makes the whole chain degrade as a whole, but why are UV rays in particular so harmful?

Don't they have less energy/photon than visible light, which would make polymers mores susceptible to degration from visible light? Or is it something like there's more UV light rays than visible light rays?

I'm confused!

Thanks!

Also, I wasn't sure to put this in the physics section or here so feel free to move it if I've made a boo boo. :)

In accordance with all of the explanations so far, the energy of the UV that creates hydrogen radicals from hydrogen gas for example, is the energy of the bond.

 

[Beasticly]

That makes a lot more sense now, but looking at a light spectrum I notice there's also xrays and gamma rays that have a higher energy again? Is it just that they are classed as UV too because they're higher than violet light?

Thanks!

In accordance with all of the explanations so far, the energy of the UV that creates hydrogen radicals from hydrogen gas for example, is the energy of the bond.

Is it possible to find the energy of a bond? I'd like to look at how additives added to polymer to make the polymer UV resistant affect the energy between bonds, so if there's some sort of theoretical calculation I can do that'll be cool, or is it something you can only do practically?

 

[Redbelly98]

That makes a lot more sense now, but looking at a light spectrum I notice there's also xrays and gamma rays that have a higher energy again? Is it just that they are classed as UV too because they're higher than violet light?

No, they are considered separate from uv. Just as radio waves are not considered infrared.

 

[Beasticly]

No, they are considered separate from uv. Just as radio waves are not considered infrared.

So does the sun not emit xrays and gamma rays or something? If not, why are plastic manufactuers so worried about UV degration if there's xrays and gamma rays that will do more damage?

 

[Redbelly98]

The sun emits a lot more uv radiation than it does x-rays and gamma rays. So while individual x and gamma photons are more energetic than uv, there are a lot more uv photons and collectively they do more damage.

 

[Beasticly]

The sun emits a lot more uv radiation than it does x-rays and gamma rays. So while individual x and gamma photons are more energetic than uv, there are a lot more uv photons and collectively they do more damage.

But doesn't the sun emit white light that is made up of light of every wavelength? Is it just that UV occupies a greater proportion of wavelengths than any other sort of light?

 

[Redbelly98]

Here is a graph showing the relative amounts of uv, visible, and ir radiation:

http://www.uvi.edu/SandM/Physics/SCI100/Downloads/SCI100MeteorologyLectures/SolarSpectrum.gif

As you can see, there is more visible and ir than uv. I was only saying the uv is more than x-rays and gamma rays, which are such a small amount that they would not even show up on this graph.

But doesn't the sun emit white light that is made up of light of every wavelength?

"White light" means there is light of every wavelength over some extended range, often just covering the visible region.

More info is at:
http://www.globalwarmingart.com/wiki/Image:Solar_Spectrum_png
http://en.wikipedia.org/wiki/Solar_radiation
http://www.google.com/search?source=ig&hl=en&rlz=&=&q=solar+spectrum&btnG=Google+Search

 

[Mapes]

Nice find, Redbelly98.