Nowadays, many methods have been developed for the separation of the different plastics that exist.
These are based on certain characteristics such as X-ray behavior, infrared spectroscopy, color differences, physical techniques, solvents and chemical markers.
The first step is the manual separation of residues in threshing plants. In these, the operators are able to separate the materials by simply observing them thanks to their experience.
But this method of differentiation is not sufficient to make a good separation of plastics by components. Poor separation of plastics can have serious consequences. This is why other methods for the separation of plastics must be used.
The X-ray fluorescence method is used, for example, to separate PVC from PET and consists of irradiating the plastic with X-rays to detect the secondary radiation emitted by the chlorine atoms in the PVC.
Infrared spectroscopy makes it possible to identify different plastics by analyzing the light reflected by the surface of the material when it is subjected to infrared radiation. Each material emits a series of characteristic radiations depending on its molecular structure.
Plastic color and opacity detection cameras are used in conjunction with infrared cameras for continuous process separation.
The above 3 techniques can be used to separate PET, PVC, HDPE, LDPE and PP, but have the disadvantage that the equipment is very expensive.
Physical techniques include hydrocyclone, triboelectric separation and density difference.
In the hydrocyclone a flow of water and plastic particles is thrown tangentially towards the top of the cone. The centrifugal force concentrates the heavier particles towards the outside, falling to the lower part where they are collected while the lighter particles move towards the upper part. This method of separation does not give very good results, being necessary to practice later another type of operation.
Triboelectric separation is based on the different electrostatic charge of small particles of polymers, caused by friction on the wall of an air vortex and where they are projected into an electrostatic field generated between two plates, to which an electric potential is applied.
The method of density difference between polymers is known as sinking / flotation, which is a system that separates a mixture of HDPE, LDPE, PP, PS and PVC. The small difference in the density value of these compounds is enough to achieve good results with this system.
The procedure consists of introducing the mixture of plastics (in pieces) in water, which contains a small amount of surfactant, so that the water wets the plastic. The polyolefins remain floating on the surface while the PS and PVC sink to the bottom.
The supernatant fraction is introduced into a mixture of lower density than water, water-alcohol. In this way the HDPE sinks and the PP and LDPE remain floating.
A similar treatment is done with the PS, PVC mixture, which is separated by means of a saline water solution.
With this method, polyolefins and polystyrene are obtained in relatively clean fractions of 97 and 95% respectively, but this is not the case for PVC, which is collected contaminated by other materials.
The procedure based on the solubility of plastics in different solvents consists of using a solvent to separate a mixture of HDPE, LDPE, PP, PS and PVC. For example, a mixture of cyclohexanoxylene can be used as a solvent, causing the separation of three different phases; one of PS, another of PVC and a third of polyolefins with purities per phase ranging between 96 and 99%. In all cases, the polymer is recovered by precipitation using a precipitating agent.
The chemical marker method consists of the specific incorporation of a certain marker for each polymer, but since the main application of the plastic is in the packaging industry, the additives to be incorporated must be completely harmless.
The use of a specific bar code for each polymer has also been proposed, but due to the diversity of sizes and geometries it is difficult to implement this method. What is in practice, and is shown in the table on the following page, is the identification of the 7 most commonly used plastics, with their respective symbols, which once printed on each article, allows them to be separated quickly and safely.
IDENTIFICATION OF RECYCLABLE PLASTICS
