From the manufacturer "Sungold Abrasives":  We start with a 65%/35% polyester/cotton blend cloth backing. This innovation in sanding belts was first introduced by Sungold in 1978.  With the market only offering 100% cotton or 100% polyester at the time, this breakthrough technology provided the strength of polyester, but employs cotton yarns to prevent the stretching that can occur with all polyester backings.  While many belts will tear or shred from nails and staples, the unique way we weave our backing creates a high tensile strength and resists tears.  A phenolic base is applied before and after the grain is applied to adhere the grain to the backing. Our proprietary high temperature resistant phenolic resins hold up to high heats generated by continuous operating on high speed belt sanders, providing extended belt life compared to other belts. The cloth is treated prior to applying the grain to create a water resistant coating on the cloth so the backing does not wilt when stored in high humidity environments.