When I got your inquiry to the group the first question I thought of was "How many different varieties of polypropylene are there?" Will there be a middle of the road or "generic" materials specification that will work?
I went on-line and found some information that could possibly assist you in creating your own materials file.
You would probably want to choose a middle of the road value for the different properties and then let a CAE specialist figure out the specifics for his or her simulations. Unless you make one part, a plastic piece will not make much difference in terms of the CG if that is the purpose of the material. In fact if you just use the density of aluminum as a base, it will probably be OK.
There are cases where a guy will go buy something like a camera and give me a 3d solid model of it. The only way to produce a good CG result is to go weigh it and then create a dummy material for it with a density that works.