Maya Ellison
There are several ways to identify different types of plastics. One common method is the float test, which involves placing the plastic in water to see if it floats or sinks. Generally, polyolefins, such as High-Density Polyethylene (HDPE) and Polypropylene (PP), float, while others, like Polyethylene Terephthalate (PET), sink. Another identification technique involves heating a stirring rod and holding the plastic sample to a flame, observing characteristics such as smoke, soot, dripping, and odour. The colour of the raw plastic can also be indicative; black or grey plastics are often thermoplastics, while yellow or tan plastics are typically thermosets. Additionally, grinding the plastic to observe if it smears or powders can help distinguish between thermoplastics and thermosets. These tests provide tentative identification, and more complex analysis may be required for definitive results.
| Characteristics | Values |
|---|---|
| Broad groups of plastic | Polyolefins generally float in water, the rest generally sink |
| Polyolefins | High-Density Polyethylene (2), Low-Density Polyethylene (4), Polypropylene (5) |
| Test to distinguish Polyolefins | Chop a small piece off a milk bottle (HDPE) and it will float; a piece from a drink bottle (PET) will sink |
| Test to distinguish LDPE and HDPE bags | LDPE makes a soft, swishing sound, HDPE makes a crisper, crinklier sound |
| Test to distinguish thermoset and thermoplastic | Heat a stirring rod to about 500°F/260°C; if the material is a thermoplastic, it will melt; if not, it is probably a thermoset |
| Test to distinguish thermoset and thermoplastic | Hold a sample to the edge of a flame; if it's a thermoplastic, it will smear; if it's a thermoset, it will powder |
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What You'll Learn
Float and burn tests can help rule out certain plastics
The float test and burn test are useful for ruling out certain plastics, although they cannot be used to specifically identify a plastic. To perform the float test, heat a stirring rod to about 500°F/260°C; if the material is a thermoplastic, it will melt, but if it is a thermoset, it will not. The burn test involves holding a sample to the edge of a flame until it ignites. Observe the nature of the smoke, the presence of soot in the air, and whether the sample drips while burning. Extinguish the flame and cautiously smell the fumes, comparing the odour to that of a known sample if possible. After performing these tests, check your observations against the known characteristics of each plastic. It is usually recommended to perform an additional test to confirm the results of the initial identification.
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Heat a stirring rod to differentiate between thermosets and thermoplastics
The fundamental difference between thermosets and thermoplastics is how they behave when heated. Thermosets are a type of polymer that undergoes a chemical reaction when heated, resulting in an irreversible change in its molecular structure. This process, known as cross-linking, creates a three-dimensional network of bonded molecules, giving the material improved strength, rigidity, and thermal stability. Once set, thermosets cannot be reheated, remelted, or reshaped, making them challenging to recycle.
On the other hand, thermoplastics maintain their chemical structure regardless of whether they are in a liquid or solid form. They can be repeatedly heated, cooled, and reshaped without altering their fundamental composition. This characteristic makes thermoplastics highly sustainable and environmentally friendly, as they can be easily remelted and remoulded into new forms, reducing material waste.
To differentiate between thermosets and thermoplastics using a heated stirring rod, one can observe the behaviour of the material when heat is applied. If the material is a thermoset, it will undergo a permanent change and will not be able to return to its original form. Thermosets will often degrade before melting when exposed to excess heat. On the other hand, if the material is a thermoplastic, it will soften and melt when heated, allowing it to be reshaped or remoulded.
It is important to note that the distinction between thermosets and thermoplastics is not solely based on their response to heat. Thermosets are typically harder, stronger, and more rigid than thermoplastics. They also have excellent resistance to heat, chemicals, and extreme cold temperatures. Thermoplastics, on the other hand, tend to have lower melting points and are more susceptible to creep susceptibility under long exposure to heat and stress loads.
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Observe the colour of the raw plastic
One way to identify plastics is by observing the colour of the raw plastic. This method is particularly useful when identifying automotive plastics. If the raw plastic is black or grey, it is generally a thermoplastic. Thermoplastics are semi-rigid plastics that typically require an adhesion promoter when being repaired. If the raw plastic is yellow or tan, it is likely a thermoset. Thermosets are flexible plastics that do not require an adhesion promoter. However, it is important to note that these colour associations are not definitive, and other tests should be conducted for confirmation.
Another test that can be performed is the float test. This test involves placing a small piece of the plastic in water and observing whether it floats or sinks. While there are exceptions, polyolefins, such as High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE), and Polypropylene (PP), generally float in water. The float test can be a quick way to narrow down the type of plastic, as if the plastic floats, it is likely one of the polyolefins, and if it sinks, it is likely something else.
The sound test is particularly useful for distinguishing between HDPE and LDPE. When crinkled, HDPE produces a crisp and crinkly sound, similar to dry leaves being squished together. On the other hand, LDPE produces a softer, swishing sound, reminiscent of green leaves blowing in the trees. PP, used in food packaging, also produces a crinkly sound but is distinguishable by its lack of stretch.
Additionally, one can lightly grind the plastic to observe whether it smears or powders. If it smears, it is a thermoplastic, whereas if it powders, it is a thermoset. These tests provide initial insights into the type of plastic but may not be conclusive for complex plastic compounds. A more rigorous analysis, including burn tests, odour identification, and comparison with known characteristics, may be necessary for definitive identification.
In summary, observing the colour of raw plastic can be a helpful initial step in identifying the type of plastic, particularly in the automotive industry. However, it is important to combine this observation with other tests and comparisons to confidently identify the specific plastic.
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Identify the plastic by its smell
Smelling plastic is one of the ways to identify it, especially if it is degrading or burning. However, it is not easy for those who are inexperienced with plastics to identify them by smell alone. A professional analysis is often required to determine the exact source of a smell.
Some people have described a strong tangy odour from cheap plastics, which is suspected to be polycyclic aromatic hydrocarbons (PAHs). PAHs are banned in commercial products in the EU and Italy. To cover up this smell, some manufacturers add a cheap orange flavouring to their plastic products.
Burning plastic has a distinctive and instantly recognisable smell. It can be caused by something as simple as a plastic utensil left on a hot stove or something more serious, like faulty electrical wiring. If you smell burning plastic, you should immediately assess whether there is any visible smoke or fire. If there is smoke or fire, or you are extremely concerned, call the fire department. Otherwise, turn off and unplug any devices, then call an electrician to determine the cause of the smell.
Plastic made from cellulose nitrate often smells like camphor.
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Grind the plastic to see if it smears or powders
To identify whether a plastic is a thermoset or a thermoplastic, you can lightly grind the plastic and observe whether it smears or powders. If it smears, it is a thermoplastic; if it powders, it is a thermoset. This test is useful for tentative identification, as some complex plastic compounds require a more rigorous analysis for identification.
When grinding plastic, it is important to note that the temperature of the material will increase, which may cause oxidation or thermal degradation. To avoid this, a coolant or cooling lubricant can be used during the grinding process to reduce the temperature. It is also important to take the necessary safety precautions to avoid inhaling particles in the air.
One method of grinding plastic is through cryogenic grinding, which involves grinding at low temperatures with the help of a cryo-mill and liquid nitrogen. This method is suitable for grinding elastic materials into a fine powder and can produce a sharp shape of powder. However, it is important to use the correct amount of liquid nitrogen to avoid excessive heating.
Commercial grinders can also be used to grind plastic into powder, and the choice of grinder will depend on the desired particle size and specific requirements. It is important to follow the necessary safety precautions when grinding plastic and to ensure that any cooling agents or lubricants used do not react with the plastic.
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Frequently asked questions
Heat a stirring rod to about 500°F/260°C; if the material is a thermoplastic, it will melt. Thermosets do not melt and are generally more flexible.
Thermoplastics are usually black or grey, while thermosets are typically yellow or tan. If you lightly grind the plastic and it smears, it's a thermoplastic; if it powders, it's a thermoset.
Plastic bags made from LDPE will make a soft, swishing sound when crinkled, while those made from HDPE will produce a crisper, crinklier sound.
Polyolefins, which include HDPE, LDPE, and polypropylene, generally float in water. So, if a piece of plastic floats, it's likely one of these.
Yes, the float test and burn test have been used for years to tentatively identify plastics. However, these tests may not always provide specific identifications.
