Whittaker brushes start with an injection molded polypropylene core for true dimensional stability and balance as well as strength. The bristles are also made of 100% polypropylene, which, unlike nylon, will not soften when exposed to moisture. Polypropylene also has a lower abrasion resistance and melting point to prevent damage to nylon carpet fibers. Whittaker brushes contain a wear indicator to let you know when it's time to change them, and are designed to last about 1 million square feet. We recommend rotating the brushes regularly to maximize the life of your brushes. Whittaker offers color-coded brushes in varying bristle stiffnesses to clean every type of carpet for each of the Smart Care® TWIN and TRIO machines.