So is rayon... kinda. It's cellulose from trees and other plants, without the original cells.

Where is the dividing line between cellulose, lignin and "plastics"?

rayon is a regenerated cellulose fibre: it biodegrades.

polyester is a thermoplastic polymer synthesised from petrochemicals: it doesn’t.

that’s the dividing line. one breaks down in the environment, the other persists for centuries and sheds microplastics into waterways every time you wash it.

rayon has its own environmental problems (deforestation, chemical processing), but “is it plastic?” is not one of them. the chemistry here isn’t ambiguous.