Low-cost identification of plastics is certainly desirable, but identifying an unknown plastic material can be an expensive exercise if you submit it for full chemical analysis.

Most of the time, we just need to know if it is high-density polyethylene (HDPE) or polypropylene (PP). Or maybe we are wondering if it is polystyrene or polycarbonate.

By methodically applying a battery of low-cost tests (density, cutting, heating, burning), allied to simple observations (transparency, hardness, stiffness) and some basic knowledge of plastics materials and associated manufacturing processes, it is possible to distinguish between different families of plastics. We can even make educated guesses about particular grades (e.g. filler content).  Heating and burning tests are carried out on tiny slivers generated during the cutting test.  The key steps are establishing whether the material is self-extinguishing or free burning, observing the colour of the flame and identifying the degradation volatiles by smell.

Using this procedure with students, some with a non-chemistry background, I found that, after 30 minutes demonstration of the tests, with appropriate attention to hazards and safe working practices, and 30 minutes practice on a set of ‘known’ samples, students could, in a further 60 minutes, identify most of a set of 16 ‘unknown’ samples, some with a high degree of confidence.

The key to the procedure is a set of ‘known’ samples. Therefore, all tests on ‘unknown’ materials are carried out in parallel with ‘known’ samples until the nearest match is obtained.

This article was written by Dr.Charlie Geddes for Hardie Polymers.

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