Plastics flame test identification

During the last years, a number of products consisting of a mixture of different plastics have made their appearance they are usually called polymer blends or polymer alloys. Their identification using simple methods presents considerable difficulties because flame tests and pyrolysis tests are usually not unambiguous. Also a separation into different groups according to the pH-value of the pyrolysates does not permit a definite conclusion. In some cases, however, it is possible to separate polymer mixtures into their components if these have different solubility characteristics and then to identify the components (see Section 6.3). 

Flammability is the susceptibility of a substance to fire or burning. This is not a property commonly used for identification purposes, but there are a few materials where the bum test is traditional. Amber, plastics, and other substances that emit a distinctive smell when heated are commonly tested with a hot needle or candle flame. This is not recommended, as it is destructive to the object and may be hazardous to the person doing the testing (the smell can only be detected from very near the object). 

The flammability of plastics can be a useful tool in their identification, but the additives present have an influence on their ability to bum. A clear test to indicate the presence of chlorine, mainly found in PVC and poly(vinylidene chloride) is the Beilstein test. A clean, copper wire 30-40 cm long, with a cork or other heat-insulating material at one end as a handle, is heated with a Bunsen burner to clean it of residual impurities, heating it until the flame is colourless. The hot, cleaned wire is placed in contact with the plastic to be analysed so that a small piece is melted onto it. The wire is returned to the flame and the colour noted a green or blue-green flame denotes the presence of chlorine while other colours suggest the plastic to be other than PVC or poly(vinylidene chloride). 

Silicones are produced in the form of resins, oils, greases, and also as rubberlike elastic products. These materials also appear as processing aids in the manufacturing of plastics, as impregnation compounds, coatings, separating materials, mold releases, etc. They can be identified because they contain the element silicon. To test for silicon, mix approximately 30 mg of the sample with 100 mg sodium carbonate and 10 mg sodium peroxide. Heat this in a platinum or nickel crucible over a flame. Dissolve the melt in a few drops of water, boil it, and then add dilute nitric acid until the solution is neutral or slightly acidic. The identification of silicon then follows in the usual way with the addition of a few drops of ammonium molybdate. (See Chapter 4.)... 

In general, simple observation and feel alone are insufficient to reveal the chemical nature of a three-dimensional plastic object. Identification schemes have been developed based on simple tests including the behaviour of polymers on impact, in liquids (flotation and dissolution) or in a flame. Such tests provide a rough, qualitative assessment of simple homopolymers but are not effective at identifying copolymers or polymer blends (Braun 1996 Cloutier and Prud homme, 1993). 

These tests are of limited value although Bird has described a simple thermal test for the identification of plastic films, (Method 105). The above tests are of limited value nowadays becaused of the wide range of polymers now being manufactured and certainly are probably of no value in the case of copolymers. Conventional low impact polystyrene is soluble in hot toluene, whereas high density polyethylene or propylene have little or no solubility in this solvent. However, if the polystyrene contains some copolymerized butadiene, as occurs in the case of high impact polystyrenes, then due to the presence of crosslinked gel, the polymer would not completely dissolve in hot toluene. So even in the case of simple polymers solubility tests are of limited value and for them to provide any useful information required detailed knowledge. Polystyrene on the other hand, unlike the polyolefins when it is held in a flame, due to its aromatic nature... 

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