A paper/cardboard cup can release particles from... its thin plastic layer into a hot drink.
This inner layer prevents the liquid from passing through the paper and helps the cup retain its shape. Researchers compared two common coatings: polyethylene, derived from fossil resources, and polylactic acid, or PLA, a plastic often described as biodegradable.
The cups were exposed to hot water, after which the released particles were analyzed using several methods. The team sought to measure their quantity, size and chemical nature. It examined microplastics, but also much smaller particles called nanoplastics.
The clearest result concerns the total mass of plastic found in the water. PLA-coated cups released approximately 11.6 micrograms per liter, compared with 0.9 micrograms per liter for those coated with polyethylene. In these tests, that represents a mass approximately 12 times higher with PLA.
The count of very small particles also produced high values. The researchers measured approximately 4.3 million nanoparticles per milliliter with the PLA cups, compared with 2.7 million with the polyethylene cups. This count detects particles, but on its own it cannot establish that they are all made of the coating plastic.
The chemical analysis provides precisely this detail. Nanoplastics originating from PLA were confirmed in two size ranges, around 60 to 130 nanometers and 200 to 300 nanometers. For polyethylene, the researchers did not detect nanoplastics using this chemical method in the fractions studied.
Why the difference? After contact with hot water, the surface of the PLA coating showed physical deformations. However, the analyses found no evidence of chemical degradation of the material. The particles could therefore be mechanically detached from the surface without the plastic being chemically broken down.
The health consequences of ingesting microplastics and nanoplastics remain poorly understood.