Most of my cats (past and present) have liked to lick plastic trash bags and grocery bags. Assuming that they are not tearing off bits of plastic and swallowing them, but just licking, is there any harm in this? What are they getting out of this -- is it just a texture thing, or are they leeching some chemical out of the plastic, or what?
Why do cats lick plastic bags?
Extensive online research shows it is not just plastic bags which appear to be cat tongue attractants, but photographs as well. The activity of bag licking varies among cats; not all cats lick bags and not all bags get licked. There is no definitive answer, but most theories support @JohnCavan's answer of bags being rendered with animal fat or having acquired smells. Unfortunately, the majority of answers found are incestuous, referring to each other.
Here is the culmination of what I've discovered, with each reason listing the seemingly independent sites which support each conclusion:
Due to the bio-degradable nature of the bag, it was rendered with something that created a lingering odor. This could be anything from rendered animal fats ("tallow"), lanolin, petrolium products, gelatin, or corn starch. As regards photographs, Manhattan Cats notes that gelatin is also used in the photo emulsion process, directly connecting photo licking to bag licking.
The Feline Express article breaks down bag materials as it relates to cats, noting the manufacturing process may have simply created an odor which we cannot detect, but cats prefer. Though Chron Blog proposes corn starch, Feline Express dismisses the ingredient, as corn starch bags were only manufactured for a short time among few vendors. Feline Express continues by dismissing tallow and even fish scales, referencing quotes from manufacturers stating they are not used.
Depending on what the bag has touched or held, there may be lingering odors from the prior contact, which the cat can detect and is trying to taste, or is simply curious about.
The coolness of the plastic, or the texture of the material, feels good to the feline tongue, and may even taste good as well.
Your cat likes the sound of the plastic bag being licked. It may simulate the noise of a caught animal. This is dismissed by some sites as unlikely since cats tend to dislike noise.
Diet / Cravings
Plastic bag licking is compensation for an unsatisfied dietary craving.
Supporting Sites: Chron Blog
Mental Health / Pica
Your cat may either have pica, the compulsion to eat things which are not food, or obsessive compulsive disorder. Either seems unlikely since the licking activity is apparently quite common among cats. HDW proposes the cat may be performing an orally compulsive act to replicate "nursing".
Is there any harm in it?
Per all the sites, you should not allow your cat to persist in licking a plastic bag, as this may tempt them to chew and swallow plastic pieces which may result from the activity. Consuming these pieces could potentially create intestinal blockage. Several of the cited sites refer to instances where plastic bags had to be removed from cats, or plastic pieces were found in excrement from the litter box. Additionally, you might not know the history of the bag; it could have contacted a substance poisonous to cats.
I recommend, when possible, that you do not allow your cat to continue licking plastic bags.
One theory I've heard is that plastic bags contain the remnants of fats from animal byproducts, which the cats can taste/smell, this seems a little doubtful, though not impossible, given that not all bags are made from petroleum products. Another theory is that the plastic has acquired scents from other substances and the cats are reacting to that.
Either theory kind of makes sense; both our cats lick bags all the time. The common connection I see is that the plastic had come in contact with things from the grocery store. This lends a bit of credence to the second theory.
I strongly disagree with the theory that cats are reacting to trace animal fats on the plastics. My personal theory is that (the cats who do this) are mistaking the plastics for fats.
If you looked at a molecular structure of most fats you'd find that most of the molecule is a chain hydrocarbon. Things like polyethylene are the same way and shorter fragments (oligomers) would possibly show up to a cats smell / taste as fats.
My problem with the animal fats theory is multiple: 1) I think most plastics are manufactured without any animal fats. For instance, mold release agents like magnesium stearate (the stearate portion could be animal fat derived or not) are not always used, bio-degradable plastics are a relatively small fraction of the plastics made (and typically they use something like corn starch, not animal fat)
2) I've seen cats do this reaction to well worn products. Thus even if magnesium stearate were used its residues should be on the surface and would have long ago been removed.
3) I typically see cats react to plastics that could conceptually be "misinterpreted" by a chemical sensor (taste bud, smell nerve) as a fat. They seem to react more to things like polyetheylene, polypropylene which would present hydrocarbon chain ends.