Black Polyester. Ideal for big hitters looking for explosive spin and a powerful punch, RPM Blast from Babolat is a co-polyester string with a fast and powerful response. As used by Rafael Nadal, Jo-Wilfried Tsonga and Samantha Stosur. High density co-polyester construction provides a fast and powerful response with great ball speed out of the frame. An octagonal structure with cross-linked silicone provides maximized spin. Cross-linked silicone coating provides a unique feel.