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What is the material used to coat (some might use the term 'lacquer') disposable aluminum containers in the food industry? And can it be removed in a safe way (for the environment) with 'home methods'?

I am not talking of the thin oxide layer that covers aluminum foil and most basic (shiny) containers; this layer can be tunneled through and a resistance measurement with an ordinary multimeter will show high conductivity even without scratching it.

I am in particular interested in thin, easily crushable containers like dessert cups (for example mildly acidic fruit mousse).

aliberico alucoat. Source: Aliberico (see below)

The coating appears different from that used in aluminum cans, but maybe it is just a different thickness.
There is no conductivity no matter how much I scratch the surface, and only when I punch through the material with the probes I get aluminum conductivity.
The label just say Alu 41 and my google-fu has been found lacking when it comes to the coating.

I googlefound the websites of two manufacturers that produce the containers like the ones I am interested in, like Aliberico, but I cannot find the name of the coating material, just a tradename (Alucoat) and some vague marketing descriptions.

This is the 'datasheet' I have found for Alucoat: https://www.paroc.com/-/media/uploaded-product-docs/2023/01/16/15/15/alucoat-en-us.ashx?dmc=1&ts=20230409t1527075538 and there is no mention of the material.

So, let's focus the question to this particular instance: what is the coating of Alucoat made of? Can it be chemically removed, for example to create small pads?

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    $\begingroup$ 'Alu41' is a recycling code and doesn't really help in identification of the coating shall there be one. I'm afraid there is no universal answer. First, and there might be no coating at all, see related question Which side of an aluminum foil should I put in contact with food? Second, rolls of foil are sometimes coated with wax (which annoys HPLC users), and the same might be happening with the containers. Third, there are numerous plastic-coated aluminium foil (e.g. Al-PP) types used industrially. $\endgroup$
    – andselisk ♦
    Commented Apr 9, 2023 at 6:47
  • $\begingroup$ Metallic aluminum is always covered by an alumina layer (aluminium oxide) that is thin, continuous, adherent, transparent and waterproof. If a piece of aluminium is scratched so as to remove this layer, it is immediately reformed by the atmospheric oxygen. So it is not necessary to add a second layer of wax or lacquer to this oxide layer. $\endgroup$
    – Maurice
    Commented Apr 9, 2023 at 8:26
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    $\begingroup$ Aluminium can for storing fizzy drinks usually have an internal coat of plastics like PET to prevent reaction between the can and the drink. Disposable aluminium "containers" rarely do as they are usually intended for cooking where most coatings would burn or char. Disposable containers are not intended for long term storage so they don't need a coating (unlike drink cans). $\endgroup$
    – matt_black
    Commented Apr 9, 2023 at 12:38
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    $\begingroup$ @matt_black the reason I am asking is because there is a coating, unlike some basic disposable containers. I can tell from the conductivity: aluminum foil and basic "silver" lasagna container have a thin aluminum oxide layer that can be tunneled through or scratched by the probes and the material is all in all very conductive. The containers I am talking about are not: there is no scratching that will make them conductive and even with 24V applied to them there is no current flow. Also, there are companies that produce these coated containers. I will update my question with links. $\endgroup$
    – Peltio
    Commented Apr 9, 2023 at 13:53
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    $\begingroup$ @Peltio I think this is a perfectly fine question for Chemistry.SE, especially after you you have provided additional info. $\endgroup$
    – andselisk ♦
    Commented Apr 9, 2023 at 16:56

2 Answers 2

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You can experiment to find out what the colored coatings are. For example, to test for lacquers and similar organic coatings (e.g., cellulose acetate butyrate, nitrocellulose, formvar, etc.), try various organic solvents (acetone, ethyl acetate, xylene, ethylene dichloride, etc.). N.B. Most of these solvents are highly flammable, and some are fairly toxic -- use in a lab hood with supervision. Formvar, for example, is not readily soluble in acetone, but is in ethylene dichloride.

Nitrocellulose lacquer can be identified by instability to heat, and the characteristic odor of nitrogen oxides as heat decomposes it.

If no solvent affects the coating, it might be colored anodized aluminum, though anodizing might be too expensive for disposable foil products. $\ce{NaOH}$ solution will dissolve the $\ce{Al2O3}$, but would not affect most organic materials in the same way (it might wrinkle a lacquer, rather than directly dissolve it).

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    $\begingroup$ Yes, the outside color of the containers I have reminds me of anodized aluminum, and so do all colors of the ones in the picture I posted. And it appears certain types of anodized aluminum do have exceptionally high resistivity: finishing.com/69/03.shtml . I will try mechanical abrasion with fine grained sandpaper first, then acetone and then if I can do it safely the other solvents. Thanks for all the links. $\endgroup$
    – Peltio
    Commented Apr 9, 2023 at 19:29
  • $\begingroup$ @Peltio, you might report your results as an answer to your question, and help satisfy my curiosity, too! $\endgroup$
    – DrMoishe Pippik
    Commented Apr 9, 2023 at 23:51
  • $\begingroup$ So far I have managed to remove the yellowish exterior coating using NaOH, but the interior, which is silverish, appears to resist the hydroxide attack (I can see small bubbles only along a few, almost invisible, scratches on the surface). I am wondering if there is an additional thin organic film on the inside or if the exterior was easier to attack due to diffused microscratches... I will report back when I find out. $\endgroup$
    – Peltio
    Commented Apr 10, 2023 at 19:11
  • $\begingroup$ FTIR analysis using a one bounce diamond cell will quickly identify the coating[s] on the aluminum. $\endgroup$
    – jimchmst
    Commented Apr 13, 2023 at 4:10
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Ok, after a few experiments and a more focused search, following Dr. Moishe Pippik's answer I have come to the conclusion that:

These cups are made of aluminum, covered with a layer of anodized aluminum and, at least on the interior, an additional epoxy film.

The yellowish exterior can be dissolved by NaOH so it seems it is just a plain anodized aluminum layer. After a few minutes of exposure to concentrated NaOH solution the thin layer dissolves (starting from microscopic scratches) and once the aluminum is exposed, the reaction takes off with generous bubbling and the formation of a white precipitate (which I assume is aluminum hydroxide).

Exterior is easily attacked by NaOH

The silverish interior is more resistant to NaOH, and that seems to validate the suggestion given in the accepted answer and the comment that some sort of protective film is added. I found confirmation on this other producer's website: https://www.ablpack.com/ where it shows the same type of packages and says that

...the epoxy coating on the surface protects the aluminum foil material from direct contact with food and prevents human ingestion of aluminum.

After I left the hydroxide inside the container for a few hours, it was able to erode the entire material at the perimeter and leak outside (in a plastic container!). This exposed the plastic film that appears to be on the inside only as can be seen in the following picture:

Plastic film is inside

Note how the exterior (anodised aluminum layer) already started to dissolve.

Therefore, I believe the mystery has been solved (almost literally).

Creation of pads is better done by mechanical abrasion with fine sandpaper and then cleanup of the surface with sodium hydroxide.

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  • $\begingroup$ Nice work! BTW, it seems you're trying to make electrical contacts or pads on aluminum. It's difficult to make long-lasting electrical connections because aluminum forms a very hard oxide coating immediately after being cleaned, and it is difficult to solder to copper wire. There are special fittings for making contact to Al wire, though:inspectapedia.com/aluminum/Aluminum_Wiring_Repair_Methods.php $\endgroup$
    – DrMoishe Pippik
    Commented Apr 11, 2023 at 0:20
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    $\begingroup$ @DrMoishePippik I have seen a method to solder on aluminum by covering it first with an alloy of tin and zinc, under silicon oil to avoid oxidation. Once that layer is formed, it is possible to solder normal tin or tin-silver solder. Not sure if it will be possible with these flimsy containers because it needs to get real hot... But for what I have in mind, simple temporary mechanical connections will suffice. $\endgroup$
    – Peltio
    Commented Apr 11, 2023 at 3:30

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