The container is being attacked

The failure that is invisible for months and then affects an entire production run at once.

The bottle is part of the formulation

A cleaning product is frequently the most aggressive liquid a consumer plastic will ever hold: solvents, strong alkalis, acids, oxidizers and fragrance oils.

The bottle is not a neutral container. It is a material in contact with your formula for the whole shelf life, and it can change and be changed.

How it shows

Paneling: the bottle collapsing inward.

Swelling: the opposite, from internal pressure or from the wall absorbing solvent.

Crazing: a network of fine cracks, characteristic of solvent stress.

Softening, so the bottle deforms in the hand.

Weight loss, which is product permeating out through the wall, and is invisible until you weigh a bottle.

A seeping neck, which is usually the closure liner rather than the bottle.

Cloudiness or a changed smell in the product, which is the container leaching into the formula rather than the reverse.

What causes it

Solvents, above all, and terpenes such as d-limonene are among the worst.

Fragrance oil, which is a solvent and is the reason a fragranced version fails when the unfragranced one did not.

Strong alkali in PET.

Oxidizers in several resins.

And heat, which accelerates every one of them.

Catching it before the customer does

Fill bottles with the real formula, cap them as sold, and keep them at room temperature, warm, and hot.

Weigh them. Weight loss is the earliest quantitative signal and it precedes anything visible by months.

Run it for at least three months, longer for a long shelf life, and repeat it when the formula, the fragrance, the bottle supplier or the closure changes.

A supplier's compatibility chart is a starting point. It describes the resin against a pure chemical, and your formula is neither.

References