I am wondering what exactly is happening on the molecular or atomic level when our sun fades the color out of fabric.
Is it entirely due to the UV portion of the light? How does that radiation degrade the pigment? Is it because its very high frequency can penetrate between molecules or what?
Do microwaves degrade pigment too?
Thanks, Dopers!
It depends on the substance which forms the pigment; different substances absorb different wavelengths. Actually, since dyes need to absorb light in the visible spectrum, I posit (1) that much of the degradation will be triggered by visible light.
1: that’s “I went to college” for WAG
Most of the damage is done by ultraviolet light. As electromagnetic radiation gets higher in frequency, part way through the UV portion of the spectrum it becomes ionizing, meaning that it can strip the electrons off of atoms and create ions. This is what damages the pigments in fabrics. Ionizing radiation is also dangerous to living tissues, and is well known to cause cell damage and cancers. There’s a reason your suntan lotion specifically blocks UV.
Visible light is too low in frequency to be ionizing. Higher frequency radiation (x-rays, gamma rays, etc) also causes damage, but we don’t get that much higher frequency radiation from the sun. The vast majority of what we get from the sun is infrared, visible light, and ultraviolet.
ETA: Microwaves are lower in frequency than infrared, so they are also too low in frequency to be ionizing.
Colored organic dyes get their color through long delocalized electron orbitals. These bonds are conjucated double/triple bonds with heteroatoms (not-carbon, usually N, S, P, O) thrown in. UV light does many reactions with these bonding systems. The primary example of these reactions is the Norrish reactions. The short of it is that UV light has enough energy to push bonding electrons into non-bonding or anti-bonding orbitals. Sometimes the electrons just relax to the original state, sometime they split and bond elsewhere.
Visible light does not generally do this, but it can in special cases.
Thanks, all, I really appreciate the detailed answers.
Hmm, let’s see, what things do I know of that stop ionizing radiation? Well, lead for one. (lead apron in the Xray booth.) Zinc oxide also, it’s that stuff lifeguards used to put on their noses.
Looking up “zinc oxide” in Wikipedia, I see that it can even be used as a coating on plastics to prevent UV color fade. What is it about these specific elements that makes them reflect this kind of radiation? Are there other substances that also have this capacity/ Did things painted with lead-based green paint (before it was banned) have greater resistance to sun fade?