Also most microplastics in the environment are from rubber tires grinding away on road surfaces so the food packaging stuff is a little forest for the trees imo.
Additionally polypropylene degrades under sunlight, becoming brittle. The process takes several months.
the most common plastics used in packaging are pet, polypropylene, and polyethylene, which have exceptional elongation at break. when you scratch them, you don't produce microplastics; you produce a burr on the surface
Anyway, they're already present in tap water, so your lettuce is also already infused with microplastics.