USA: +1-585-535-1023

UK: +44-208-133-5697

AUS: +61-280-07-5697

59.7 CHEMICAL TESTS ON PLASTICS

Identification of an unknown sample of a plastic, rubber or a fiber may be required for a variety of reasons. Many of these polymers carry their trade name only and their detailed structure is not” disclosed. In this age of competition, a manufacturer may like to develop a product similar to that of another producer. In such cases, it may be necessary to analyse the polymer sample and identify it. Some common methods are discussed as follows:

1.      Visual Examination. A visual examination of the sample may reveal useful information. For example, the. presence of a hard, inflexible flash line would indicate a thermoset moulded material. Similarly, the presence of a gate scar would indicate an injection moulded material. The physical form of the sample, i.e., whether granules, film, sheet or fiber and its flexibility or rigidity would give some indication of its identity. The colour of the product can also be used for identification of a thermoset material. For example, a pastel shade would rule out a phenolic and probably indicate” that it is a urea or melamine-formaldehyde plastics material.

2.      Cutting. The cutting of a plastics material with a pen knife can also provide some information. For example, one can differentiate between crystal clear cellulose acetate and poly (styrene). Thermoset and thermoplastic material can also be differentiated in this way. The ivory like cut of casein and cast phenolics is noteworthy.

3.      Heating tests. A small amount of the material to be tested is taken in a spoon type of spatula and heated on a small bunsen flame. The ease of burning, whether the burning continues after removal from the flame, colour of flame and so on, all give an indication of the possible identity of the material. If the material explodes or burns away rapidly, it is possibly a cellulose nitrate composition.

A second heating test can be conducted with the help of a clean copper wire. The wire is first heated in a clear: bunsen flame and then touched with a small quantity of the material, It is heated again and the colour of the flame is noted. Blue and green colours indicate the presence of halogens in the composition, that is, chlorine, fluorine and rarely, bromine. Presence of poly (vinyl chloride) or its co-polymers, poly (vinylidine chloride), Polyttetrafluoroethylene), chlorinated rubber, rubber hydrochloride or cellulose acetate containing a plasticizer like tricresyl phosphate is indicated by this test.

A third heating test is carried out by heating a small sample in a hard glass tube. The gas evolved is condensed in another tube and very carefully smelled. This can then be compared with the gas evolved from a known polymer.

4.      Fusion Test. The metallic sodium fusion test, can show the presence or absence of nitrogen and halogens. Similarly, the potassium nitrate/potassium carbonate fusion test will indicate the presence or absence of phosphorus in the material under examination. Thus, one can easily distinguish between Nylon and Terylene.

 Identification of Polymers

From the preliminary tests, a great deal of information can be gathered regarding the possible identity of the unknown polymer. If the polymer does not show rubber like elasticity,
an elastomer is ruled out. Further, if on heating it does not melt or flow, a thermosetting polymer is indicated. If it does melt, a thermoplastic polymer is indicated.