Mixing types and grades of polymers can ruin a batch of product, so knowing the type of polymer is important. While most recyclable plastics are labeled with a code number, some are not; moreover, shredding can destroy the code number. While FTIR can identify polymer types, DSC and TGA are needed to determine grades (HDPE versus LLDPE for example) and amounts of filler respectively.
Full analysis of an unknown may require:
Chemical composition by FTIR
Glass transition and Melting point by DSC
Filler content by TGA
Overlay of Grade 2 and 4 PE