Both Polypropylene and Polyester are synthetic materials that are resistant to absorbing liquids. As a result, they are "solution-dyed," with each fiber being colored and then woven instead of being colored once made into a rug. Both materials are excellent for keeping their colors, even in the sun. They are both lightweight and mold, mildew, and stain-resistant. Polypropylene will be easier to clean and will wear less than Polyester rugs.