Why does plastic bottle goes very thinner when lemon juice is stored in it over a couple of days or weeks?

Answer 1

Below is a "best guess". I'm not certain whether or not either of the two explanations are correct, but they're the most sensible explanations I can think of.

Lemon juice contains a weak organic acid.

The acid will react with the plastic polymers, which will undergo "acid hydrolysis". The acid acts as a catalyst, while the water in the lemon juice reacts with the polymer and breaks it down into its monomers.

For example, if a bottle were made out of PVC, it would be made of a long chain of lots of these:

H Cl

-C-C-

H H

The acid hydrolysis reaction breaks the long chain up into an alcohol (chloro-ethanol).

This reaction is very unlikely to occur with PVC due to its stability (it is used here for its simplicity), but other plastics may undergo acid hydrolysis quite easily (for example, polylactic acid, which is used as 'biodegradable' plastic).

Acid hydrolysis would only occur where the plastic meets the lemon juice, so only the inside surface would be hydrolysed. This would explain why your bottle feels thinner and weaker, since the hydrolysis has actually thinned the plastic.

Alternatively, any plastic left in strong sunlight may become weaker over time. This will be due to thermal decomposition in extreme cases, but mostly it will be because warming the plastic breaks the intermolecular forces that hold the long polymer chains together. This allows them to 'flow' more freely, which would explain why it feels thinner. In this case it wouldn't be thinner at all, but would give that impression as it would feel softer and bend more easily. Once cooled, the plastic may "set" again in its position, and stiffen, but some plastics will not set so easily, and may remain flimsy forever.

This effect is the same principal as ironing - the heat allows the structure to change easily, then sets in its new position once cooled. Ironing also relies on breaking intermolecular forces that hold individual molecules together.

It is possible, but I cannot confirm it, that the acid in the lemon juice will cause this effect to happen faster or more easily, but it should not occur at all (by this method) if the bottle is kept out of direct sunlight and away from radiators or other sources of heat.