As you would expect high quality rubber. Plastic screws to secure to fender will probably use ss-screw / washers with locking nut for better piece of mind. This size mud flap is perfect for recumbent trikes front fenders as it doesn't seem to bottom out. I currently have a HP Velotechnik Scorpion FS26 and up graded to these SKS mud flaps, because the ones that came with the fenders are riveted on and are not as thick even though they are SKS fenders being used by HP Velotechnik on their trikes.
I would definitely buy these again for my trike and have no problem recommending them.