Nova Chemicals Arcel is a mix of polyethylene and polystyrene. Developed for bodyboards in the mid-80's, this is a light, strong foam used for bodyboard cores. It is a tough, flexible, durable foam that is stronger and more water resistant than polyethylene. Arcel maintains its stiffness even in warmer water but has a tendency to dent. Polypropylene Polypropylene is light, highly responsive, strong, springy, waterproof closed cell foam core.
Polypropylene is much stronger than polyethylene, with half the weight.
New lamination techniques seem to have rectified this problem. Polypropylene comes in two types. Extruded polypro is made from strands of polypropylene and offers very good projection and durability in the board. Beaded polypro is a step up and is extremely lightweight, durable, water-resistant, and as a result, expensive, but you get what you pay for. It is generally accepted that polypropylene is the premiere core material in the market.