The Perfect Bungee Cords are made of 100% polyeurethane, not rubber. They have molded nylon ends that won't scratch. They also carry a lifetime guarantee (when used as directed) from breaking, cracking, or fraying and are resistant to fuels, US, ozone, saltwater, and most household chemicals. The coreds have a stretch range of two times their original length and will retain flexibility and shape over a wide range of temperature extremes.