Thermoplastic materials can be reused by various recycling processes, the most important step being the sorting of the materials. Laboratory tests can be used to determine the components of an unknown material. Some very simple test methods are shown below in order to give the necessary guidelines to easily identify the basic types of thermoplastic polymers.
Polymer resins can be identified by the following fundamental tests:
- Standardized marking
- Physical appearance
- Heat effects
- Belstein test
- Solubility
- Relative density
Standardized marking
The Society of the Plastics Industry (SPI) introduced the code for resin identification in 1988 in the interest of recycling and pollution abatement and to establish a uniform system for the entire United States.
The codes are molded or printed on the bottom of most plastic containers, however, for large-scale sorting, visual identification is not practical enough.
Physical appearance
The physical or visual appearance can provide a clue to identify plastic materials. It is more difficult to identify plastics as unmixed raw materials, or pellets, than finished products. Thermoplastics are generally produced as pellets, granules. Thermoset materials are usually obtained as powders or resins.
Of the most common thermoplastic resins, polyethylene terephthalate (PET) and polypropylene (PP) have a translucent, waxy texture.
The methods of manufacture and application of the product also describe a plastic. Typically, thermoplastic materials are extruded or subjected to injection molding, calendering, blow molding and vacuum forming. Polyethylene, polystyrene and cellulosics are commonly used in the container and packaging industry. Substances such as polyethylene, polytetrafluoroethylene, polyacetals and polyamides have a characteristic waxy feel. Thermosetting plastics are usually compression molded (transfer molded).
Effects of heat
When plastic samples are heated in test tubes, the characteristic odors of certain plastics can be identified. The particular way they burn can give a clue to this.
PET burns quickly, giving off kerosene odors and, when the flame is extinguished, produces a white smoke.
Polyethylene burns with a transparent blue flame and drips as it melts.
Polyvinyl chloride (PVC) can ignite but will itself extinguish as soon as the source of the fire is far away. PVC has a very sour smell when burning because hydrogen chloride is a burning derivative.
PP, on the other hand, burns more slowly, gases feed the flame.
Polystyrene (PS) and its copolymers give off black smoke (carbon), burns quickly, has a strong gas odor, and produces large amounts of soot.
The actual melting point is another identifier. Thermoset materials do not melt. Some thermoplastics, on the other hand, melt at less than 195ºC. It is also possible to press down on the surface of a plastic with an electric welding torch. If the material softens and the hot tip sinks, it is a thermoplastic. If it remains hard and carbonizes, it is simply a thermoset.
Beilstein test
The Beilstein test is a simple method to determine the presence of a halogen (chlorine, fluorine, bromine and iodine). For this test, a clean copper wire is heated in a Bunsen flame until it becomes incandescent. The hot wire is then quickly brought into contact with the test sample and the wire is returned to the flame. A green flame shows the presence of halogen.
Chlorine-containing plastics such as polychlorotrifluoroethylene, PVC, polyvinylidene chloride and others, which test positive for halogen. If the test is negative, it is possible that the polymer is composed only of carbon, hydrogen, oxygen or silicon.
Solubility
Tests to determine the solubility or insolubility of plastics are simple methods of identification. With the exception of polyolefins, acetals, polyamides and fluoroplastics, all thermoplastic materials can be considered to be soluble at room temperature, PET for example is impervious to chemical solvents while PP dissolves in hot toluene. Thermosets, on the other hand, are resistant to solvents.
Relative density
Density classification with a flotation system is performed by comparing the densities of the plastic material and liquid substances of known density. If a plastic floats in a solution with a density of 0.94 g/cm3, it may be a medium or low density polyethylene plastic. If the sample floats in a solution of 0.92 g/cm3, it may be a low density polyethylene or polypropylene. If it sinks in all solutions below a density of 2.00 g/cm3, the sample will be a fluorocarbon plastic.
The presence of fillers or other additives and the degree of polymerization can make it difficult to identify plastics by relative density, as they can cause the density of a plastic to change quite a bit. Low density polyolefins, ionomers and polystyrenes will float on water (which has a density of 1.00 g/cm3).