I don't know the exact details on your PP. For example, is it opaque or colored? It makes a big difference when analyzing.
The first thing is your FTIR analysis. The base spectrum for PP is pretty distinct. If you have extra peaks, you might try subtracting
a base PP spectrum from your unknown and then running it through your data base. It could give you a lead on a few additives.
The major identifier on PP is molecular weight. I don't know how much sample you have, but there are a couple of ways to identify it.
GPC is one way. Since it is PP, it takes a high temperature GPC and some nasty solvents. But it is possible. If you have a large amount
of sample and it seems to be PP only (no special additives), you can try a test like melt index. It gives an idea of processing characteristics.
With PP, the minor additives help a lot. You might try a soxhlet extraction followed by GC/MS to see what additives are present (foe example,
BHT).
If you have fillers added, try EDXRF and ash. If the ash gives a fair amount of residue, you may be able to run it by FTIR and id the additive.
EDXRF will identify elements present (i.e. Si and Al would suggest clay, which you should be able to confirm by IR).
The best way might be a lab that specializes in polymer analysis.
good luck.