is a trademark for chlorosulfonated polyethylene (CSPE) (CSM) which is a type of synthetic rubber.  It is noted for its excellent resistance to chemicals, temperature extremes, oxidizing chemicals, concentrated acids and caustics, oils, sun, UV light,  puncturing and abrasion.  Since Hypalon coated boats are extremely resistant to environmental caustics, they are highly recommended for use in severe climates.   For boaters who plan on leaving their boats inflated, or for those who plan on using them frequently Hypalon is the choice.  It is not susceptible to swelling.  Its maximum temperature limit is 212º F. ( PVC's limit is 130º F )