Today we did some tests regarding the identification and separation of plastics. In most cases plastic waste is mixed with other kinds of plastic. For the recycling process it is important to have a single kind of plastic. There are multiple techniques available to identify plastics like Near Infrared Spectrometry, the burn test and electrostatic separation. The most simple technique however, separate the plastics by density, requires just water and salt and can be performed in your own kitchen.
Some plastics have a lower density than water (e.g PP 0,92 kg/dm3) and other have a higher density (e.g. PS 1,06 kg/dm3). By cutting them in small pieces and putting them in water they separate by density. This way it is for example possible to separate the PE caps from the PET bottles (PE 0,96 kg/dm3 vs PET 1,39 kg/dm3). And PP from PS and PLA (see pictures above). By increasing the density of the water by adding salt it is possible to increase the density of water and make heavier plastics float. By adding 10 grams of salt to 100gram water the density is now 1,10 kg/dm3 and now PS starts to float.
The density of water can also be lowered by mixing it with alcohol, which has a density of 0,79, so the lighter plastics can be separated as well.
Plastic densities (kg/dm3):
Check out this YouTube video by Recoup to see the test in action!