How does ink permeate through plastic bags?

Submitted by DANDYDORF t3_xuox64 in askscience

I’ve had many horror stories happen at our vinyl shop where permanent marker/standard sharpie can leech through the bag, tape it was written on, and ruin the vinyl inside within a day or so. One time includes when a coworker had a large roll of vinyl, wrote size on paper transfer tape, and stuck that to the outside of the bag the roll was in. It ruined the banner entirely. For this, we strictly do not write on plastic with sharpies here. Why does this happen? Is it something about the oils? Why can’t i find any info about this?

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Smyley12345 t1_iqwri4n wrote

Markers use solvent as a means to transfer ink. Depending on the type of tape, marker, and plastics involved I would suspect marker solvent damaging the plastic. Many types of plastic are not very solvent resistant.

By careful selection of your marker, your tape, or your type of bags you could probably avoid this but a "no markers on plastic"rule accomplishes the same thing without requiring much technical know how.

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TheThingsWeMake t1_iqwruep wrote

Most "permanent" inks have an evaporating solvent in them to keep it liquid as you write, then become permanent when dry. Sometimes these solvents can eat through other materials. It is also why a dry erase marker can remove Sharpie from some surfaces, the solvent in the dry erase dissolves the Sharpie ink.

TLDR: the ink is melting through the other material a tiny bit.

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doctorandusraketdief t1_iqwsymi wrote

This phenomenon is called diffusion. When you use a waterproof marker you can't wipe it off with water because the solvent and the dye of your marker are apolar substances. Water a polar and doesn't mix (well) with apolar substances. Polymers however are apolar as well. So even though vinyl isn't a liquid it's made of long apolar chains (which I always imagine like spaghetti) and those polymer chains have the same apolar properties like the dye and solvent in your waterproof marker, so they mix well. Over time the dye is simply absorbed into the vinyl. If you would take a water based dye this will not happen.

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TeenyTinyScientist t1_iqxjvzs wrote

I just want to add that despite how they're sometimes presented, plastics, especially thin cheap plastics, are actually pretty permeable. They are typically fairly resistant to water intrusion so people often assume they are good barriers, but that's not accurate. Polymers like these plastics have pores and substances can diffuse through them.

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