The one that will stay with you the longest on your outdoor journey may not be your partner, but the sun! Compared to the nylon and cotton fabrics popularly used in the market, polyester is more resistant to UV rays and less likely to be deformed or discolored by prolonged exposure to sunlight. Some backpacks that use nylon or cotton materials perform far worse in this regard than those that use polyester fibers, but they don't let that reduce their selling price.