I just looked on the Pellon website. Here's a quote from their FAQ section:
"Q: What is scrim binder?
A: Scrim binder is a thin sheet of polypropylene which is very much like a dryer sheet that is needle punched onto one or more sides of batting as it is processed. Not all scrim binders used today are alike. Legacy™ By Pellon batting uses only a very lightweight scrim. This adds strength and durability that is sought by machine workers yet allows softness and ease of stitch that everyone appreciates. Our scrim does not create heavy, dense and stiff results."
http://www.pellonideas.com/category/55-faqs.aspx