When we say plastic to describe materials, it is the moldability characteristic of the polymer materials, with long chains of molecules that are polymerized hydrocarbons. The porosity of a polymer is a function of many things and yes, some are very porous. Goretex is a plastic menbrane that is waterproof, but allows individual water molecules tho pass through. So it makes great rainwear, waterproof on the outside, breatahable to get rid of evaporated body water (sweat) from the inside. Another example is regular polyethylene plastic wrap vs Saran wrap. Polyethylene passes flavors through where Saran blocks them. That's why Saran works so much better for longer term storage and that's why grovery stores usually don't use polyethylene wrap - it allows the scenting agents from things like laundry detergents to penetrate fresh food, especially meat, giving the off flavor of something that comes from a supermarker.
