Rounded polypropylene replacement vent covers with old style curled hinges fit older Ventline 14" x 14" Roof Ventadomes and Elixir universal camper vents. Polypropylene is a polymer that is sturdier than other plastics and serves double duty as both a plastic and a fiber. Over time, other plastics fatigue from the repetitive stress of being opened and shut, and eventually will break. Polypropylene is very resistant to this sort of stress, and is the plastic most often used for lids and caps which require a hinging mechanism. Ventline RV vent lids are constructed of Polypropylene to bring you years of problem free use.