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How can I remove labels from jars?

Sometimes I like to recycle jars for my own use, but normally I want to remove the label first. I find this extremely difficult to do. With ordinary stickers, like the stickers found as price tags on books, acetone works very reliably.

However, the labels on food jars are stuck on with a much more tenacious glue that does not respond to vegetable oil, acetone, methyl ethyl ketone, hot vinegar or ethanol. What is this stuff and how can I dissolve it?

Right now the only way I have found is to laboriously scrape them off with razor blades, but it is extremely time consuming to do this.

(Note that the tough labels I suspect are "hot melt" adhesives used to affix polypropylene labels. For example, traditional hot melt compositions are ethylene/vinyl acetate copolymer and ethylene/(meth)acrylate copolymers, but obviously there are others. I am hoping somebody with expertise can identify the range of typical compositions and corresponding solvents.)

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    $\begingroup$ Related question on Arts & Crafts: How do I remove labels from glass jars/bottles? $\endgroup$ – Loong Mar 17 '17 at 6:55
  • $\begingroup$ @Loong I have seen that. I am more interested in a scientific answer, not folklore from arts and crafts people. $\endgroup$ – Shaka Boom Mar 17 '17 at 7:03
  • $\begingroup$ Based on your deduction that the adhesive is of the types you mentioned, you could try isopropyl alcohol (or methanol if you can be safe about it). I am surprised acetone didn't work, my only reason to suggest an alcohol is that they are slightly more polar and that could work with the acetate/acyrlate portion of the glue. $\endgroup$ – J. Ari Mar 17 '17 at 13:26
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I think this question is impossible to answer. There are dozens of chemical types for label adhesives. Most are thermoplastic, meaning they don't form new covalent bonds with the surface. But some are thermosets, meaning they react and cross-link.

In general, it is quite difficult to dissolve a crosslinked polymer since the dissolution process requires destruction (scission) of covalent bonds. The ways to do that are obvious: heat, oxidation, reduction, acids, or bases.

One thing that needs to be said here is consideration of both the opportunity cost as well as the environmental cost of your method generally makes all but the easiest methods inadvisable. When you say the adhesive "does not respond" to X, do you soak the label in X? (I typically use a slightly larger container (say part of a PET bottle) and a shallow layer of the solvent and allow it to soak overnight.

It's always a good idea, in my opinion, to start with water with a drop or two of surfactant in it. If hot water doesn't work, I move on to 70% IsoPrOH and give that a few hrs. Next, I use an alkaline (basic) detergent solution and give that another overnight soak. If it's worth my time, I end with an acetone soak (I have acetone which I reuse for this purpose in its own container.) One of these works most of the time. When none of these work, I toss (recycle) the jar. You may or may not know that not all polymers are soluble. So, your assumption that there is a solvent for the label you want to remove isn't necessarily true.

The most aggressive solvent I am aware of is mixtures of methylene chloride, dodecylbenzene sulfonic acid, and N-methyl pyrrolidone. That was used to de-pot Soviet microelectronic back in the Cold War days to reverse-engineer them. Anyway, glass is mostly Si-O-Si bonds and alkali bases should loosen any organic's adhesion to it.

Be sure to remove any surface layer on the label - abrasion is generally quickest. Oh, speaking of dangerous chemicals (methylene chloride is a carcinogen, and ddbsa is corrosive to the skin), I've also used aqueous HF to clean glass. The HF actually removes some of the silica, but if done carefully enough doesn't visibly damage the glass surface (never would try this with glass jars; why bother? but with expensive glass flasks, and other apparatuses, it was worth the hassle. The disposal of the waste solvent was a pain, though.)

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