ABS sidewalls are the most popular material for making ski and snowboard sidewalls.  ABS plastic is very durable and water resistant that makes it a good option for skis, snowboard, and even splitboards.  It allows flexing of the snowboard while maintaining it’s properties.  It also adheres well to other materials and acts as  vibration reduction.

Warning, this video is a bit weird, but if you want to know more about ABS, it will help.  Plus, the guy has a sweet mustache.