UV degradation of different plastics?

[kenny1999]

I have been searching on Google about UV degradation of plastics , polypropylene in particular. I understand fully that it will break the plastics and so many plastics / polypropylene are not suitable for usage under sunlight. I also found that on the molecular level the sunlight will break the bond and release free radicals. However, I can hardly find any information about what would be the end --products of the UV degradation of plastics. Is the end-product released into the air bad for human? I have a lot of PP containers which are so big in size and they are often stressed under sunlight in daytime in a small room which is not very well ventilated. Would this accumulate some gaseous plastics circulating around the room or other unhealthy chemicals ?

 

[HAYAO]

I don't know much about PP, but typically, it is hard to identify the end product of photo-degradation.

There are several reasons to this, for example, it depends on the atmosphere, it depends on the exact polymer (molecule) in question, it depends on what wavelength. Also, a reaction, particularly things like radicals, may show unpredictable behavior. However, at the same time, the reaction may be not enough to identify and measure quantitatively with typical measurements (IR, NMR, MS, etc.); in principle they can be detected, but the likelihood is that they will be buried by the signal of the main component.

I don't think the photo-degradation of PP is strong and active enough to produce dangerous chemicals (although you still want to treat it with caution).

 

[chemisttree]

The end product will be oxygenated plastics. In the IR you would look for the -OH hump, COOH carbonyl, ketone carbonyl and ether C-O stretches. Those free radicals react with oxygen on the atmosphere. If you were analyzing the plastic itself, you would look for chain length changes. You would see an increase of dispersity (polydispersity) in the MW. Not many volatiles would be released but volatile byproducts are certainly a possibiliiy.

 

[kenny1999]

The end product will be oxygenated plastics. In the IR you would look for the -OH hump, COOH carbonyl, ketone carbonyl and ether C-O stretches. Those free radicals react with oxygen on the atmosphere. If you were analyzing the plastic itself, you would look for chain length changes. You would see an increase of dispersity (polydispersity) in the MW. Not many volatiles would be released but volatile byproducts are certainly a possibiliiy.

If a Polypropylene made thing (or other plastics) is never exposed to sunlight, it would still break the bond but the rate of reaction is slower or negligible, am I correct? If a Polypropylene made thing (or other plastics) is exposed to sunlight for a moderate period of time but it is then kept dark or unexposed , would the degradation reaction still proceed easier/faster because some bondings were already broken before, the structure of the plastic was already weakened, and thus it's easier / faster to break the rest of the plastics even if it's kept dark. Is my Chemistry concept right or wrong??

 

[chemisttree]

I would agree with that. Just what “moderate” means and if you could actually measure it would be another thing altogether.

These plastic containers are not pristine in nature. In their manufacture they were heated to melt and then blow molded or extruded into final shape. This action alone causes chain scission, oxidation cross linking and so forth. Degree of crystallinity, residual stresses also play a role. Regions that have higher residual stresses will degrade more rapidly as will the less crystalline amorphous phase of the intercrystalline matrix. Rate of cooling from melt and strain rate during cooling can seriously affect crystallinity as well. UV isn’t the whole story.

 

[kenny1999]

I would agree with that. Just what “moderate” means and if you could actually measure it would be another thing altogether.

These plastic containers are not pristine in nature. In their manufacture they were heated to melt and then blow molded or extruded into final shape. This action alone causes chain scission, oxidation cross linking and so forth. Degree of crystallinity, residual stresses also play a role. Regions that have higher residual stresses will degrade more rapidly as will the less crystalline amorphous phase of the intercrystalline matrix. Rate of cooling from melt and strain rate during cooling can seriously affect crystallinity as well. UV isn’t the whole story.

Sorry I understand that the whole picture is not that simple. However, in general, is Polypropylene safe for human if I work and live with it for long period of time under normal stress (sometimes it may expose to sun , heat ). I am not worried that they are getting damaged or cracked, I am worried that they are releasing unknown chemicals. Am I overreacting?

 

[Borek]

is Polypropylene safe for human if I work and live with it for long period of time under normal stress

Yes.

 

[DaveE]

Polypropylene safe for human if I work and live with it for long period of time under normal stress

Safer than driving a car, climbing stairs, sitting in front of your computer all day, or eating lots of junk food. Of course "safe" is a relative thing.

 

[HAYAO]

Sorry I understand that the whole picture is not that simple. However, in general, is Polypropylene safe for human if I work and live with it for long period of time under normal stress (sometimes it may expose to sun , heat ). I am not worried that they are getting damaged or cracked, I am worried that they are releasing unknown chemicals. Am I overreacting?

You might not want to lick it if you leave it out outside for years, but it should be safe if you are going to work with it indoors. There's a good reason they are used for containers.

 

[kenny1999]

Hi now. I have another question. Is colored PP container usually less "safe" than transparent PP container? My guess is they need to put more addictive into the plastic to make it colored, and thus more likely to leach addictive over time. Am I correct? My Chemistry knowledge is not very much.

 

[chemisttree]

Chemical safety and health effects of colorants aren’t really chemistry either. That’s biology or epidemiology or health care stuff.
Testing for the bad stuff, IS chemistry (once its been identified).

There are hundreds or perhaps thousands of compounds used in colorants for plastic and so there are hundreds of answers to the question of colorant safety.

Lots of “food grade” dyes have been found to be hazardous over time. When I was younger, red dye#3 (FD&C Red #3 to be exact) was being investigated as harmful and was eventually removed (as it’s “lake”) from the food grade list. Most modern dyes, maybe all of them, pose no risk from simple contact or inhalation. That’s inhalation of the dye when employed in final form , not the guy adding a lake powder to a batch of something during manufacture. Still there are reports of compounds containing heavy metals like lead, cadmium and chromium being used as pigments for paints on Children’s toys!

The answer to your question can only be, “Maybe.”