InStyle: Plastic Vs Nylon Mascara Brush

"Plastic is preferred for lengthening and separating. However, because that style grabs onto less formula than its nylon counterpart, a few more coats might be needed to get the coverage you want. Nylon bristles pack it on to build luscious volume. While not always the best at separating, they can deposit a thick coat from corner to corner for speedy swipe-and-go situations."