Like avalanches and whiskey distilling, ski boot construction boils down to a science. In this case, designers tinker with a plastic's chemical properties to balance durability, density, temperature stability, and cost.
About 70 to 80 percent of alpine boots on the market are made of polyurethane (PU), a hardwearing and beefy plastic that dampens vibration, absorbs shock, and is easily stretched or ground. This makes for an aggressive, smooth ride. But PU is also heavy.
Enter polyamides (PA). Good for cruising uphill, they're around 20 percent lighter than PU and chiefly used in touring boots. For reliable flex, they behave uniformly across a range of temperatures. Downside: PAs aren't as stiff or damp on the descent. Pebax is a popular, rock solid choice, while Grilamid is the boot-makers' new secret sauce. Stiffer than Pebax, this nylon plastic is robust but pricey.