Take about 5 mg of your sample and grind it into a fine powder. This step may take some time and some muscle power. This is probably where you are having difficulty making a powder out of your plastic samples. So best to spend some time and grind the "hell" out of your sample so it came be formed into a homogeneous wafer with KBr. Using a rather sensitive balance, weigh about 0.5mg of the sample and blend with either KBr (KBr is preferable) or CSI to homogeneity. You can use a small mortar and pestle of a dental mixer (makes better blends) for this step. Then you need a dedicated IR pellet press, either a small hand version which you can purchase from most any chemical supply house, or a small industrial press with a round, steel, dedicated pellet form. Use from 50 to 100 mg of the blend to make a small round, hard, rather thin wafer. Your FTIR should already have a pellet holder, which is dedicated to holding your thin disc for data collection.