Question: How do I identify the base polymers in an unknown plastic for recycling?

profpearce is asking a question about spectrometry
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by profpearce | May 30, 2019 04:47 | #19564

Has public labs spectrometry work ever been used to identify the base polymers in an unknown plastic for recycling? Anything like an OS FTIR in the right range ready for show time? Thank you for your help - Joshua Pearce


Check the "baby legs" projects on microparticles. It will give some background. My own experience was in medical plastics industry. We routinely used FTIR, and variations, to ID plastics. For example, we would use microscope FTIR, a variety of ATR methods, and all kinds of different preps( solvent cast, pyrolysis, hot press film, etc.). Usually, this will ID the plastic, but occasionally we would have to rely on GPC, HPLC, EDXRF, or GC/MS. The other methods would help identify fillers and antioxidants, as well.

#babylegs is the link... Thanks!

Actually I've seen @maxliboiron post pics of plastics inside of plankton, fluorescing, but I don't know that it would make plastic type identification possible. Max, may I share that photo from your presentation?

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With PVC, as the plastic degrades, you get more and more conjugated double bonds. Hate to get chemical, but the where the plastic absorbs starts going from IR to NIR to visible. But it happens all at once. In clear plastic, it will change from clear to yellow to black.
Flourescing could show that.

@warren : The fluorescing plastics you mention are from specially designed lab plastics that floresce, so not related to polymer type or to everyday plastics. FTIR and other spectrometry is notoriously difficult with marine plastics because they pick up so many other chemicals in water, so the spec profiles are usually different from library spectrums of virgin plastics. Chelsea Rochman at University of Toronto is working on a project to make a library of marine plastic spectrum profiles, but chances are that this won't be universal. Even with high-end equiptment, this is tricky. More citizen science methods to ID polymers are via relative density by floating them in water, salt water, oil, etc, and using burn tests and hot needle tests. To my knowledg, there's no comprehensive and validated forensic guide on that... something to work on for us!

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You know, I think I remember someone trying this with #hyperspectral imaging.

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Density is difficult to use. Many manufacturers use fillers like titanium dioxide as a filler. This will dramatically alter the density, depending on the amount added. As for the burn test, yes this works. It is a variation on the pyrolysis method mentioned earlier. Something like polystyrene burns with a Smokey flame, while polyethylene burns with a much cleaner flame. Again, the additives can dramatically change this. By identifying the pyrolysis by FTIR, you can get an idea of what you are looking at. Pyrolysis is more often used with elastomers than plastics. But hot press can often be used with plastics.

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