Currently, many methods have been developed for separating the various types of plastics that exist.
These methods are based on certain characteristics of the plastics, such as their behavior under X-rays, infrared spectroscopy, color differences, physical techniques, solvents, and chemical markers.
The first step is the manual separation of waste at sorting plants. There, operators—drawing on their experience—are able to separate materials simply by observing them.
However, this method of sorting is not sufficient to properly separate plastics by type. Improper sorting of plastics can have serious consequences. That is why other methods must be used to sort plastics.
The X-ray fluorescence method is used, for example, to separate PVC from PET and involves 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 from the material's surface when it is exposed to infrared radiation. Each material emits a series of characteristic radiation patterns based on its own molecular structure.
Cameras that detect the color and opacity of plastics are used in conjunction with infrared cameras for continuous sorting.
The three techniques described above can be used to separate PET, PVC, HDPE, LDPE, and PP, but they have the drawback that the equipment is very expensive.
Physical separation techniques include the hydrocyclone, triboelectric separation, and density differentiation.
In the hydrocyclone, a stream of water and plastic particles is directed tangentially toward the top of the cone. Centrifugal force causes the heavier particles to be concentrated toward the outside, where they fall to the bottom and are collected, while the lighter particles move toward the top. This separation method does not yield very good results, so another type of operation must be performed afterward.
Triboelectric separation is based on the different electrostatic charges of small polymer particles, caused by friction against the wall of an air vortex, where they are propelled into an electrostatic field generated between two plates to which an electric potential is applied.
The method based on differences in polymer density is known as sink/float separation, a system that can separate a mixture of HDPE, LDPE, PP, PS, and PVC. The slight difference in the density of these compounds is sufficient to achieve good results with this system.
The process involves placing the mixture of (shredded) plastics in water containing a small amount of surfactant, so that the water wets the plastic. The polyolefins float to the surface, while the PS and PVC sink to the bottom.
The supernatant fraction is added to a mixture with a lower density than water—a water-alcohol mixture. As a result, the HDPE sinks, while the PP and LDPE float.
A similar process is used for the PS-PVC mixture, which is separated using a saline solution.
Using this method, polyolefins and polystyrene are obtained in relatively pure fractions of 97% and 95%, respectively; this is not the case for PVC, which is collected contaminated with other materials.
The process, which is based on the solubility of plastics in different solvents, involves using a solvent to separate a mixture of HDPE, LDPE, PP, PS, and PVC. For example, a mixture of cyclohexane and xylene can be used as the solvent, resulting in the separation of three distinct phases: one consisting of PS, another of PVC, and a third of polyolefins, with phase purities ranging from 96 to 99 percent. In all cases, the polymer is recovered by precipitation using a precipitating agent.
The chemical tracer method involves the specific incorporation of a particular tracer for each polymer; however, because plastic is primarily used in the packaging industry, the additives to be incorporated must be completely harmless.
The use of a specific barcode for each polymer has also been proposed, but due to the wide variety of sizes and shapes, it is difficult to put this method into practice. What is currently in use, however—and is shown in the table on the next page—is the identification of the seven most commonly used plastics, along with their respective symbols, which, once printed on each item, allow for their quick and safe sorting.
IDENTIFYING RECYCLABLE PLASTICS
