Ethan Clarke
Identifying unknown plastics is a complex process that often requires laboratory testing by experts. However, there are some basic techniques and tests that can be used at home to tentatively identify different types of plastics. It is important to note that some of these tests, such as the burn test, should be approached with caution due to the toxicity of plastic combustion products. Before performing any tests, it is recommended to first check for a label, as most plastic products are labelled for post-consumer recycling using the SPI resin identification coding system.
| Characteristics | Values |
|---|---|
| SPI resin identification coding system | Check for a label |
| Density test | Check density against reference liquids of known densities |
| Burn test | Observe flame colour, smoke, nature of smoke, presence of soot, dripping, smell of fumes |
| Beilstein copper wire test | Indicates the presence of chlorine |
| Acetone test | Most plastics become "tacky" on exposure |
| Stress test | PVC turns white when bent |
| Thermal behaviour test | Expose polymer to heat and observe behaviours such as softening, melting, burning, and charring |
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What You'll Learn
Check for a label
The first step in identifying a piece of plastic is to check for a label. Since 1988, the SPI resin identification coding system has been widely used by plastics manufacturers to label their products for post-consumer recycling. The SPI recycling codes are a useful basic taxonomy of the polymers that one is likely to encounter.
If the plastic has no label, you can move on to other tests to identify it, such as the float test, burn test, Beilstein copper wire test, or density test. However, checking for a label should always be the first step, as it is the safest and most convenient way to identify a plastic.
The burn test, for example, can be dangerous due to the toxicity of many plastic combustion products. It is also not always effective in specifically identifying a plastic, although it can be useful for ruling out certain types. To perform a burn test, hold a sample of the plastic to the edge of a flame until it ignites. Note the colour of the flame, the nature of the smoke, the presence of soot, and whether the sample drips. Then, extinguish the flame and cautiously smell the fumes, comparing them to the known characteristics of different plastics.
The float test is another simple method of identifying plastics, as plastics with different densities will float or sink in water. However, this test may not always be accurate, as plastics can have varying densities, and other factors such as thickness and temperature can affect whether a plastic floats or sinks.
Therefore, checking for a label is the best initial step in identifying an unknown plastic, as it is a safe, convenient, and accurate method that can save time and reduce potential risks associated with other identification methods.
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The float test
Polyethylenes and polypropylenes, for example, float in water due to their low density of 1 g/mL. In contrast, most other plastics will sink. By performing this test, you can quickly determine if your plastic sample is one of the few types that float or if it belongs to the denser category.
For further discrimination between plastics that float, you can use liquids with different densities, such as glycerin (density = 1.26 g/mL). ABS and PS plastics, which are denser than water, will float in glycerin. On the other hand, PETE and PVC plastics, which are even denser, will sink in glycerin.
While the float test is a useful initial step in identifying unknown plastics, it may not always provide a definitive answer. It is most effective when used in conjunction with other tests, such as the burn test or density comparison with reference liquids, to narrow down the possibilities and confirm the exact type of plastic.
Additionally, it is important to handle plastics with caution and avoid direct contact or inhalation of fumes during testing, as some plastics may release toxic substances. Always refer to safety guidelines and use appropriate protective equipment when conducting these identification tests.
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The burn test
The observable properties that can be used to identify plastics through the burn test include whether the material will freely burn in air when a gas ignition flame is removed. The colour of the flame and the smoke produced can also be used as indicators. For example, ABS tubing is identified by its characteristic flame colour and smoke when burned.
Density tests can also be used to identify plastics. If the sample is made of a single material, its density can be checked against a set of reference liquids with known densities by dropping the sample into a small vial of each liquid. If the sample floats, it is less dense than the reference liquid; if it sinks, it is more dense. Polyethylenes and polypropylenes, for example, float in water (density = 1 g/mL), while most other plastics sink.
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Beilstein copper wire test
The Beilstein test is a simple test used to screen organic and polymeric materials for the presence of chlorine. It is suitable for a wide variety of products, including plastic films and sheets, adhesives, rubbers, coatings, solvents, and fabricated items.
The test involves heating a copper wire and putting it on the material being tested. The presence of chlorine in the material will cause the normally colourless or slightly blue flame to turn a brilliant green or blue-green colour. This colour change is due to the reaction of chlorine with copper compounds at high temperatures, producing excited, green-coloured copper atoms or ions.
To perform the test, use a copper wire thick enough not to melt too quickly, such as 12- or 14-gauge copper wire, stripped of its insulation. Heat the wire to red-hot and then quickly bring a fragment of the sample near the flame so that the smoke produced envelops the wire. A green flame surrounding the copper wire indicates the presence of chlorine.
It is important to note that some plastics, such as cellulose nitrate, may burst into flame during the Beilstein test. This reaction is usually considered a positive test for cellulose nitrate. Additionally, the test should be performed in subdued lighting to easily observe the colour of the flame. The wire should also be cleaned between tests by washing it with water and dilute nitric acid to remove any materials that may cause unwanted colouration.
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Susceptibility to acetone
When identifying unknown plastics, susceptibility to acetone can be a useful test. Most plastics will become "tacky" when exposed to acetone, except for polyethylene and polypropylene.
Acetone can have varying effects on different plastics. While some plastics are resistant to acetone, others may be severely affected, especially at high concentrations and over long periods. The chemical makeup of plastics that are similar to acetone will be damaged by it. The surface of the plastic may soften, smear, or even dissolve.
Some plastics, such as PTFE (polytetrafluoroethylene), commonly known as Teflon, are known for their chemical resistance to acetone. It is transparent, maintains clarity, and is available in various thicknesses, including sheets. Another option is to use a clear plastic sheet with a chemical-resistant thin film applied, such as Makrolon AR2, which has >24-hour resistance to acetone.
Before choosing a plastic for custom parts, it is important to consider its susceptibility to acetone and other chemicals. Consulting a plastic chemical resistance chart can help in selecting a suitable material for your product's longevity.
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