Chloe Zhang
Plastic is everywhere, from water bottles to combs, and it's essential to recycle it properly for the planet's well-being. With various types of plastics, each with distinct characteristics, it can be tricky to identify them accurately. The most common method is to look for the international recycling symbol, three arrows forming a triangle with a number in the middle, which indicates the plastic type based on its chemical makeup. Another simple method is the float test, which differentiates plastics based on density. However, this method isn't foolproof on its own and is often combined with other tests like the burn test, which, despite being less safe, can help identify plastics by the unique odours they emit when burned. Additionally, plastics can be distinguished by their appearance, feel, and response to heat.
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What You'll Learn
Float test
The float test is a simple method to identify different types of plastics by their density. Different plastics have different densities, so they will either float or sink in water.
The test is useful for ruling out certain plastics, but it is difficult to specifically identify a plastic using this method alone. It is often used in conjunction with other identification methods.
To perform the float test, simply place a piece of plastic in water and observe whether it floats or sinks. Polyolefins, which include High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE), and Polypropylene, will generally float in water. On the other hand, plastics like Polyethylene Terephthalate (PET) and Acrylonitrile Butadiene Styrene (ABS) will sink.
It is important to note that the float test may not always provide a definitive identification of the plastic type. For example, PVC and PET have similar densities, so they cannot be easily separated using the float test. Additionally, the appearance, feel, and smell of the plastic can also provide clues about its type.
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Burn test
One way to identify plastics is by performing a burn test. This test has been around for years and is a good way to rule out certain plastics, although it is difficult to specifically identify a plastic this way. Here is how you can perform a burn test:
- Cut some strands of the plastic you want to test and hold them with a pair of flame-resistant tongs.
- Ignite the plastic strands over a flame-resistant pan. Be sure to perform this test in a well-ventilated area.
- Observe the flame and the behaviour of the burning plastic. Different types of plastics will burn differently and have distinct smells. For example, PVC has a sweet, aromatic smell when burned, whereas PE does not drip while burning and has a sweet, waxy odour.
- Note the colour of the flame and the behaviour of the melt. Some plastics will have a distinct flame colour, and some will produce a smoke that can be an indicator of the type of plastic.
- Extinguish the flame and examine the residue. The burn test can help identify plastics based on their behaviour during the test and the residue they leave behind.
It is important to note that this method can be dangerous and should be performed with caution and proper safety equipment.
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Odour
Some plastics have a distinctive odour, such as polystyrene, which has a lingering smell similar to burnt styrene. Polyvinyl, on the other hand, does not have a strong smell when burnt, but it can irritate the nose with the stench of chlorine. Polyethylene burns with a blue flame and yellow tips, and has a distinctive odour, while PVC plastic burns with yellow and green flames.
Some plastics, particularly cheaper products, have a strong tangy odour. This is often due to the addition of an odourising substance to cover other smells, usually indicating the presence of an ester. The intensity of the odour can indicate the concentration of VOCs (volatile organic compounds) in the plastic.
There are other ways to test plastics, such as the density test, which involves placing a piece of plastic in water to see if it floats or sinks, or the burn test, which identifies plastics by the colour of the flame and the smell they produce when burnt. However, burning plastics can be harmful to health and the environment, so it is not recommended.
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Appearance and feel
The appearance and feel of plastic can help identify its type. However, it is important to note that some complex plastic compounds require rigorous analysis for identification.
Polyvinyl Chloride (PVC) has a soft, tough, and sometimes sticky feel. It is often yellowish and has a glossy texture. It is also one of the most hazardous plastics and was once the second-most used plastic after polyethylene. Polyethylene (PE) has a wax-like and smooth texture and is flexible but can leave marks. Low-Density Polyethylene (LDPE) is thin and can crumble in the hands with a loud crackling noise. High-Density Polyethylene (HDPE) is tougher to deform and has a translucent, milky white coating. Polypropylene (PP) is bendable but does not break easily. It feels smooth and firm, and unlike polyethylene, it does not stretch. It is commonly used for food packaging.
Polystyrene is brittle and can break easily. It has a distinct smell of styrene. Nylon (PA) appears milky white and gel-like, with a hard surface that feels warm to the touch. Acrylonitrile Butadiene Styrene (ABS) is off-white or cream-coloured, opaque, and amorphous. It is tough, hard, and not easily broken. Polymethyl Methacrylate (PMMA) is transparent, can be bent when heated, and is hard to shatter.
Additionally, the density of plastics can be determined through the floating method, where plastics are placed in water to observe their buoyancy. The float test, burn test, and pyrolysis test can also help identify plastics by observing their combustion characteristics, such as odour and flame behaviour. Another simple method is to look for international recycling codes, usually found at the bottom of containers, which indicate the plastic type based on chemical makeup.
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Chemical makeup
Plastics are identified by unique codes based on their chemical makeup. The simplest way to identify a plastic is by locating three arrows forming a triangle with a number in the middle. These international codes offer clarity on plastic identification based on chemical makeup and grading for recycling. For example, Symbol 1 refers to polyester and PET plastics, which are the most common plastic types found in households, including clothing, plastic bottles, and insulation materials. Symbol 2 represents High-Density Polyethylene (HDPE), which is strong and resistant to acidic chemicals.
Another method to distinguish plastics is by their density, or gravity dressing. Different plastics have different densities, so their buoyancy in water can be used to identify them. However, this method is usually used in conjunction with other tests for accuracy. For instance, PP and PE can be separated from PET using water, while PP, PE, PS, PA, ABS, and PC can be identified using a saturated aqueous solution of calcium chloride.
The float test and burn test are also traditional methods to rule out certain plastics, although they may not provide specific identification. Additionally, the appearance and feel of plastics can be used for preliminary identification. For example, PE feels smooth, soft, and tough, while PP feels smooth and is not easily marked by scratches.
The smell of burning plastic can also provide clues to its identification. For example, PET smells like burnt sugar, PVC has a chlorine-like odour, LDPE and HDPE smell like candle wax, and ABS and polystyrene smell like styrene.
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Frequently asked questions
The simplest method to identify different types of plastic is by distinguishing their appearance, feel, and density. For instance, PE feels smooth, soft, and tough, whereas PP feels smooth and is not marked when scratched. The floating method can also be used to test the density of plastics by observing whether they sink or float in water.
Polyolefins generally float in water and have the same flame and dripping "firebomb" effect. They also smell similar to candle wax.
The international plastic identification code uses symbols with three arrows forming a triangle and a number in the middle. For example, Symbol 1 represents polyester and PET plastics, while Symbol 2 represents High-Density Polyethylene.
PVC, or polyvinyl chloride, feels soft and tough with a sticky texture. It is commonly used in soles, toys, wire sheaths, doors, windows, stationery, and packaging containers.
ABS plastic feels hard, tough, rigid, and is not easy to break. It appears milky white or beige, opaque, and dull. It sinks in water and produces a crisp sound when struck. ABS is commonly used in home appliance casings, mechanical parts, helmets, and suitcases.
