I don't think its polypropylene, The FWT is LDPE, low density polyethylene. Those are milky white in color typically. Opaque black colored tanks are generally ABS, I suppose it could be LDPE with carbon black in it.
If its polyethylene it can be difficult to adhere to but not impossible. ABS is easier. Hobie Kayaks are polyethylene and I repaired some cracks in mine successfully. I first stop drilled the cracks, then sanded the cracks and surrounding surfaces. Then, very important, I flame treated them, which activates the surfaces and greatly improves adhesion. You just wave a propane torch over the area. Then I used gflex epoxy from West Systems. Good stuff, much more flexible than standard epoxy. My cracks were in high stress areas and stayed closed.