Maya Ellison
Plastics are all around us, and identifying them is a useful skill. There are several ways to identify plastics, including appearance, touch, and even smell. The most common plastics in our daily lives include polyethylene, polypropylene, polystyrene, and polycarbonate. Each has distinct characteristics: polyethylene is flexible and smooth, polypropylene is bendable but hard to break, polystyrene is brittle, and polycarbonate is associated with baby bottles and food cans. The appearance of plastic can also be a clue: polyethylene is often translucent and milky white, while polypropylene is usually colourless and waxy. A simple way to identify plastic is by looking for a triangle of arrows with a number in the middle. A more destructive method is the burn test, which can be dangerous but can help identify the type of plastic by the colour of the smoke and the smell it gives off.
| Characteristics | Values |
|---|---|
| Appearance | Smooth, flexible, stiff, white, flimsy, fragile, brittle, thin, translucent, waxy, bendable |
| Sensation | Can leave marks, crumble, crackle, make a loud noise, resistant to fatigue |
| Markings | Three arrows forming a triangle with a number in the middle, angle brackets, e.g. >< >ABS<> |
| Burning smell | Paraffin, chlorine, sweet, like burning hair, epoxy |
| Solvent reaction | Dissolved by acetone, methyl ethyl ketone, 2-butanone, methylene chloride |
| Float test | Sinks or floats in water |
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What You'll Learn
Look for the plastic type's symbol
One of the easiest ways to identify the type of plastic is by locating three arrows forming a triangle, often with a number or abbreviation in the middle. This is known as a resin identification code or resin code and indicates the type of plastic the product is made of.
For example, Polyethylene terephthalate, or PET, is a type of plastic that is commonly used for packaging foods and liquids like water bottles and soda. It is identified by the number "1" inside the triangle. High-Density Polyethylene, or HDPE, is another type of plastic often used for containers of juice or shampoo. It is identified by the number "2" inside the triangle.
Other types of plastics may be marked within angle brackets, such as "PC" for Polycarbonate and "PA6" for nylon.
It is important to note that not all plastics have a standard symbol or code, and some may only have abbreviations or brand names printed on them. In these cases, other methods such as visual inspection, feeling the material, or even a burn test can be used to help identify the type of plastic.
Additionally, the appearance and feel of plastic can provide some indication of its type. For example, polyethylene plastics have a smooth texture and are flexible, while Low-Density Polyethylene (LDPE) is thin and can crumble easily, often making a loud crackling noise. High-Density Polyethylene (HDPE), on the other hand, is tougher and more resistant to deformation, with a translucent and milky white coating. Polypropylene is highly flexible and resistant to fatigue, making it ideal for product containers. Polystyrene, on the other hand, is brittle and can break easily.
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Feel the material and assess its texture and flexibility
One of the easiest ways to identify plastics is by feeling the material and assessing its texture and flexibility.
Polyvinyl Chloride (PVC) products feel soft, tough, and sometimes sticky. They have a glossy texture and often appear yellowish. PVC is also one of the most hazardous types of plastic.
Low-Density Polyethylene (LDPE) feels soft and smooth. It is thin enough to crumble in your hands and makes a loud, crackling noise. LDPE also produces a soft, swishing sound when rubbed together.
High-Density Polyethylene (HDPE) feels harder and more crinkly than LDPE. Plastic shopping bags made from HDPE produce a crisp, dry, leaf-like crinkling sound when scrunched.
Polypropylene (PP) packets sound similar to HDPE but are even stiffer and do not stretch. Instead, they rip and tear when pulled.
Polystyrene is brittle and can break very easily.
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Listen for a crackling noise when handled
One way to identify whether an object is made of plastic is to listen for a crackling noise when handled. This is a clear sign of low-density polyethylene (LDPE). LDPE is a type of plastic that is thin enough to crumble in your hands, producing a crackling noise. It is flexible and can leave marks. On the other hand, high-density polyethylene (HDPE) is tougher to deform and is, therefore, less likely to make a crackling noise. HDPE usually has a milky white or waxy appearance.
Another way to identify LDPE is by its smooth texture and flexibility. It is also commonly used in plastic bags, which can be identified by their translucent appearance. LDPE is often used in the production of plastic bags due to its low cost and ease of processing. However, it is important to note that LDPE is not as durable as other types of plastic and can be easily torn or punctured.
In contrast to LDPE, HDPE is a denser and more durable form of polyethylene. It is often used in the production of plastic containers, such as milk jugs and detergent bottles. HDPE is also widely used in the automotive industry for fuel tanks and other components. This type of plastic is known for its high impact resistance and good chemical resistance.
Polyethylene is a common type of plastic that can be identified through a combination of visual inspection, tactile feedback, and auditory cues. By feeling the material and listening for a crackling noise, one can distinguish between LDPE and HDPE. LDPE is more flexible and prone to cracking, while HDPE is stiffer and more durable.
It is important to be able to identify different types of plastics for various reasons. For example, certain plastics may be safer for food storage than others. Additionally, knowing the type of plastic can help with proper disposal or recycling. LDPE and HDPE, for instance, have different recycling streams and end uses due to their distinct properties.
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Perform a float test
One way to identify the type of plastic is by performing a float test. This method is commonly used to distinguish between ABS and Polypropylene. ABS sinks in water as it has a specific gravity (SG) of 1.06, whereas Polypropylene would float, with an SG of 0.946. However, it is important to note that the presence of bubbles or voids in an ABS sample could impact the accuracy of this test, as it may cause the ABS to become buoyant.
The float test is a simple and non-destructive way to narrow down the type of plastic, but it may not always provide a definitive answer. It is a useful initial step in plastic identification, but other tests or methods may be needed to confirm the specific type of plastic.
To perform the float test, you will need a container of water and the plastic object or sample you want to test. Ensure that the plastic sample is clean and dry before starting the test.
Start by filling a container with water, preferably one that is large enough to allow the plastic sample to be fully submerged without hitting the sides or bottom. Gently place the plastic item into the water and observe whether it sinks or floats. If it sinks, it may be ABS or another plastic with a similar specific gravity. If it floats, it could be Polypropylene or a plastic with a lower specific gravity.
While the float test is a helpful tool, it should be noted that some plastics may have additives or fillers that can affect their buoyancy. Additionally, the size and shape of the plastic item can also impact the results. Therefore, it is always a good idea to combine the float test with other identification methods, such as visual inspection, feeling the material, or performing a burn test, to confirm the specific type of plastic.
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Burn a small sample and analyse the smoke
Burning a small sample of the unknown material and analysing the smoke can help identify whether it is plastic. This test should only be used for tentative identification, as some complex plastic compounds require a more rigorous analysis.
Firstly, cut a small piece of the material, around 2.5 cm or 1 inch in length. It is not necessary to cut the plastic into a small piece if you are holding it in the flame. Next, hold the sample to the edge of a flame until it ignites. If no flame is produced immediately, hold it in the flame for about 10 seconds.
Once the sample is burning, observe the nature of the smoke. If it is ABS plastic, you should see black smoke with a sooty quality. Other types of plastic may produce a green, blue, or orange flame. Note whether the smoke has no colour, or whether it has a yellow flame with blue edges, which would indicate ABS plastic.
After extinguishing the flame, cautiously smell the fumes. ABS plastic has a very strong, acrid, pungent, and rubbery smell when it burns. If the plastic is giving off a stinging smell that irritates your nose, it is likely ABS. However, be very careful not to inhale too much as some plastics can be toxic when melted.
Finally, compare your observations to the known characteristics of different plastics to make a tentative identification. It is usually good practice to perform an additional test to confirm your results.
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Frequently asked questions
The easiest way to identify plastic is by locating three arrows forming a triangle with a specific number in the middle. This number indicates the type of plastic.
You can identify the type of plastic by its appearance and feel. For example, polyethylene has a smooth texture and is flexible, while polystyrene is brittle and can break easily.
Yes, one method is the float test. Different types of plastics have different specific gravities, so some will float while others will sink.
You can differentiate between these two plastics by applying a small drop of solvent on the surface. If the plastic softens and dilutes, it is ABS.
The burn test is another method used to identify plastics. It involves burning a small scraping of the plastic and then smelling the smoke. Different plastics will smell differently when burned. For example, polyolefins will smell like candles, while PVC will smell like chlorine.
