There is another thread on this topic, which I can't take the time to search for now. But I attached the following MSDS sheet there as well, which I got off of a supplier's website:
http://www.sbs-frp.com/products.htm
It is vague about the type of polymer used, but my guess it that it is a polyolefin mxture, probably polypropylene. But that is just my guess, I am not an expert on this material.