Identifying plastics

Plastics can be qualitatively and quantitatively analysed using different methods. A whole series of simple, effective methods exists for daily application which will allow a plastic type to be approximately identified. With some experience, materials can frequently be quickly categorized using the listed methods. However, in the case of modified plastics, determination is more complicated. In this case, only laboratory-supported tests can help with material identification.

Enclosed please find some more detailed information on the identification of plastics.

Unmodified materials can be visually assessed on the basis of the differences in their structure and of the resulting effect on light permeability. Amorphous plastics such as PC, PSU, PPSU etc. are transparent or translucent. Semi-crystalline plastics such as PE, POM, PA, PET, PPS, PEEK etc. in contrast are mainly opaque.

The float method can be used to identify certain product groups on the basis of their density. Here, parts made of the material are placed in water or in a saturated saline solution and their float behaviour assessed.

Using the scoring method, the so-called fingernail test, soft, notchable plastics can be identified. Only PE and PTFE can be notched using a fingernail. This method allows these material types to be easily identified.

If a plastic is wetted using a strong solvent (such as acetone), a distinction quickly becomes apparent between amorphous and semi-crystalline plastics. Semi-crystalline plastics are generally insensitive to solvents and if left exposed for a short period generally show no traces. Most amorphous plastics, in contrast, begin to dissolve after only a short exposure period or develop tension cracks after a few moments.

Plastics can be easily identified by their fire behaviour (burns, extinguishes, cannot be ignited, drips), the flame colour (orange, blue, not visible), soot behaviour (no sooting, marked sooting) or fire odour. These methods are popular, as a relatively precise identification is possible given a certain amount of experience.

The table below provides an overview of the fire behaviour of different plastics.

Some additives can be determined using simple methods. Two simple examples are provided here.

There are many other analytical methods for plastics, although most of these are laboratory supported. In its own in-house laboratory, Ensinger has access to in-depth methods such as DSC, TGA, melt point determination, hardness testing etc.

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