The scientists investigated the origin of the microplastics found in drinks packaged in glass bottles with caps. Given their characteristics, they concluded that these particles probably came from the metal caps, and more specifically from the paint that covered them. The first clue: the microplastics found in the drinks were mostly the same colour and had the same composition as the paint on the caps. The second clue: the paint on these caps had tiny scratches that were invisible to the naked eye and had probably been caused by friction between the caps when they were stored before use. This friction, which released particles from the surface of the caps, was thought to be the source of the microplastics found.

The article doesn't go into detail about how the microplastics are going from the exterior caps to the interior of the bottle? The cap should be sealed.