Determination of PE coating

[01Mercy]

Hello,

At our plant we tried to wash PET with a PE coating (+/-10mu).

With an oven test (melts) we tried to prove that the PE was removed or not, since the PET will melt and the PE will degrade in a brownisch colour.

The oven test however showed some inconsistent results.
We want a better (but simple) way to be very sure if the PE has been removed from PET or not.

Thanks for any input.

Kind regards

 

[Demon3]

PE doesn't dissolve in almost any solvent. How do you plan to "wash" it off?

You can detect the PE by FTIR, surface energy (wettability) and ironically solubility. If you have a PET particle covered in PE you can't dissolve the PET using a solvent for PET because it's coated in insoluble PE.

If the PE is removed then you can dissolve the PET.

Chris DeArmitt - PhD FRSC CChem

 

[01Mercy]

Thanks for your input Demon3,

I see I didn't clearify it's about PET flakes.

We wash it in a lye/detergent solution with a certain particle load that will rub the flakes.

We tested the PE laminated (i got the info that it's a laminate instead of a coat) PET flakes on lab scale in this solution. But now we are struggling to find a quick way to find out if the PE is washed or rubbed off the flakes.

Unfortunately we don't have sophisticated measuring equipment like an ATR.

The supplier said that the PE laminate will turn loose at 70 degrees in water. But I wonder if I will really see PE residue after filtering the wash solution after washing the PE laminated flakes.

Demon3, do you think that if the PE is still on the PET flake and I dissolve the PET that I will see the PE laminate float in the solution. Since if there is still PE on the flake the PET will still dissolve because of the cut edges. But I'm not sure if I can see such a thin transparent film float.

 

[patprimmer]

PET will absorb a class of dyestuff called dispersed dyes. PE will not.

Regards
Pat
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[Demon3]

Yes, I do think that if you dissolve away the PET, any remaining PE will float.

Another density test would be to use a salt solution.

PET has density around 1.4 g/cm3 and PE is around 0.95 (depending on type). PE bonded to PET will be in between.

So, if you make a salt solution with density around 1.39 g/cm3 then PET will sink and any PE/PET flakes will float. This is cheap and simple.

Building on Pat's excellent solution you could look for a dye that stains PE and not PET. Not really sure what that would be but Lycopene (the stuff that makes tomatoes red) may work.

Chris DeArmitt - PhD FRSC CChem

 

[patprimmer]

Chris

It's pretty hard to dye PE. I know some things will stain it a bit, but it is generally considered to be impossible to dye.

Regards
Pat
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[Demon3]

Agreed. I do know that PP (very similar to PE) is stained by lycopene. That's why once your Tupperware has had tomato sauce in it, it's red and stays that way.

Chris DeArmitt - PhD FRSC CChem

 

[patprimmer]

Does it also stain PET. I don't know.

Regards
Pat
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[Demon3]

Nor do I. Lycopene is pretty low polarity so I am hoping not as PET is fairly polar. It doesn't stain SAN and other food containers made of more polar plastics. If I had the time I'd store some tomato puree in a PET bottle and see. Perhaps Mr. Mercy will.

Chris DeArmitt - PhD FRSC CChem

 

[patprimmer]

It sure is a pretty simple experiment

Regards
Pat
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[btrueblood]

Tomato juice containers (PET) don't have visible stains after drinking the product - but I don't know if they ever see high temp's like cooked/warm sauce in a food container.

 

[01Mercy]

Hi all,

Thanks for the support.

We are going to try it first with a clear wash solution just above 70 degrees, to check as the supplier said if the laminate will turn loose.
We wil check for floating material and if we can't see that, we will filter the wash solution to check for any PE remains.

If we don't see anything I will try to dye the PET with the dispersed dyes.

I don't know if lycopene will dye pet, certainly not a pet bottle. This is because most of the bottles are multy layered and you would have to be lucky to have a PET bottle with only a inner layer like nylon but much bottles have an inner and outer layer of an other material than PET. These layers are on the bottles to act as barriers for oxiden to get in or carbondioxide to go out etc.etc.

 

[TomDOT]

I am puzzled by your furnace test description - PE has a variable melting point depending on the grade, but is usually in the 105-135C range. PET is more like 250C.

You should be able to just use a 150-200C furnace and see if there is melted PE.

Have you verified your furnace test by trying it with scrapings from a known-PE-coated pipe? or are you trying to heat up the whole pipe?

If the furnace test works, all you really need is a small oven, pocketknife to take surface scrapings from the pipe and small oven-safe dishes for heating.

Alternately, you could train yourself to smell the difference between burning PE and burning PET. I know one paint chemist who can take a brief whiff from a can of paint and tell you the solvent blend. Much faster than the GCMS, though only semiquantitative.