Omar Khalid
Plastic products often have a resin identification code imprinted on them, usually at the bottom, which is a number surrounded by a chasing arrows symbol. However, this is not always the case, and sometimes, it's necessary to identify plastic without a code. The float test and burn test are two methods that can be used to identify plastics without codes. The float test involves checking whether a plastic sample floats or sinks in water. Polyolefins, for example, generally float in water. The burn test involves observing the smoke and smell produced when a plastic is burned. For example, PVC has an acrid smell like chlorine, while LDPE and HDPE smell like candle wax.
| Characteristics | Values |
|---|---|
| Float Test | Polyolefins float, most other plastics sink |
| Burn Test | - PET smells like burnt sugar or candy floss - PVC smells acrid like chlorine - LDPE and HDPE smell like candle wax - Polypropylene smells like candle wax with a hint of paraffin - ABS and polystyrene smell like styrene, with ABS having a faint rubbery smell - Nylon smells like burning hair - Epoxies smell like a burnt-out electric motor |
| Sound | LDPE makes a soft, swishing sound; HDPE makes a crisp, crinkly sound |
| Feel | PP (polypropylene) feels firm and stiff, and does not stretch |
| Markings | - ABS is often marked as >ABS - Polycarbonate is marked >PC - Nylon is marked >PA6 - Glass-reinforced nylon is marked >PA6-GFxx (xx representing the percent of glass by weight) |
| Flame Colour | - A blue flame with a yellow tip indicates polyolefins or nylon - A yellow flame with a green tip indicates PVC - A yellow flame with dark smoke could be PET or polycarbonate |
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What You'll Learn
Float test: Polyolefins float, most other plastics sink
The float test is a simple method to help identify different types of plastics without a code. This test is based on the principle that plastics have varying densities, and those with a density lower than that of water will float, while those with a higher density will sink.
Polyolefins, which include High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE), and Polypropylene (PP), typically float in water. These plastics are commonly used in products like milk bottles (HDPE) and plastic bags (LDPE). On the other hand, plastics like Polyvinyl Chloride (PVC), Polystyrene (PS), and nylon tend to sink. PVC is often used in plumbing pipes and medical tubing, while polystyrene is used for disposable cups and food containers.
The float test is a useful tool for ruling out certain plastics, but it may not always provide a definitive identification. For example, while polyolefins generally float, they can be challenging to distinguish from each other using this method alone. Additionally, some plastics, like ABS, may float or sink depending on the liquid used in the test.
It's important to note that the float test should be conducted with caution. When performing the test, it is advisable to use a small piece of plastic, such as a thumbnail-sized portion, to avoid any potential hazards. Additionally, other tests, such as the burn test or the Beilstein copper wire test, can be used in conjunction with the float test to help identify specific types of plastics more accurately.
While the float test is a simple and accessible method for preliminary plastic identification, it is not always conclusive. For a more comprehensive understanding of plastic types and their properties, it may be necessary to employ multiple tests or utilize specialized equipment.
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Burn test: PVC smells like chlorine, PET like burnt sugar, LDPE/HDPE like candle wax
A burn test can be used to identify plastic without a code, but it is not foolproof and may not work for all types of plastics. It can also be dangerous, as it may release harmful fumes or chemicals.
To perform a burn test, use a candle, lighter, and tweezers or pliers. Hold a small piece of plastic with the tweezers or pliers and burn it with the flame. Observe the colour of the flame and the smell of the smoke.
PVC, or polyvinyl chloride, will have an acrid smell like chlorine when burned. Stay away from the smoke and gas given off by PVC.
PET, or polyethylene terephthalate, smells like burnt sugar when burned. Some people liken the odour to eating candy floss or sugar candy.
LDPE, or low-density polyethylene, and HDPE, or high-density polyethylene, smell like candle wax when burned. LDPE is soft and flexible and used for plastic bags, wraps, films, squeeze bottles, and coatings. It makes a soft, swishing sound when crinkled. HDPE is stiff, durable, and opaque and is used for milk jugs, detergent bottles, shampoo bottles, toys, pipes, and buckets. It makes a crisper, crinklier sound when crinkled.
Polypropylene, or PP, smells similar to candle wax but with an element of paraffin. It has a blue flame and smells like burning oil or gasoline. It is tough and heat-resistant and used for yogurt cups, margarine tubs, microwaveable containers, bottle caps, straws, and automotive parts.
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Sound test: LDPE sounds soft, HDPE crisp
LDPE and HDPE are two types of polyethylene, a widely used plastic with a variety of applications, including film packaging, electrical insulation, containers, and piping. LDPE, or Low-Density Polyethylene, is characterised by its highly branched structure, which results in a less compact molecular structure, lower density, and greater flexibility when compared to HDPE. This increased branching leads to irregularly packed molecules and weaker intermolecular bonds, giving LDPE a softer and more flexible quality.
HDPE, or High-Density Polyethylene, on the other hand, has more linear chains, allowing its molecules to pack more closely together. This results in a higher density and a crisper, crinklier sound when manipulated. Think of the sound of dry leaves being squished together.
One of the easiest ways to distinguish between LDPE and HDPE is through a sound test. If you have a piece of plastic, try crinkling it in your hands. If the sound is soft and swishing, like green leaves blowing in the trees, you likely have LDPE. If the sound is crisper and crinklier, like dry leaves being squished together, then it is probably HDPE. This distinction is especially useful when trying to differentiate between plastic shopping bags, as many are made from either LDPE or HDPE.
It is important to note that while LDPE and HDPE have distinct auditory characteristics, they also differ in other ways. LDPE, for example, has a lower tensile strength but greater ductility, allowing it to be stretched without breaking. This makes it ideal for plastic bags and films. HDPE, meanwhile, has a higher melting temperature and greater crystallinity, making it a more rigid and sturdy option for products like milk bottles.
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Markings: Look for bracketed codes, e.g. >ABS
One way to identify plastic without a code is to look for bracketed codes, such as ABS for Acrylonitrile Butadiene Styrene plastic. This is a type of thermoplastic with a wide range of applications, from computer keyboards and Lego bricks to automotive components and protective headgear. ABS plastic is known for its toughness, impact resistance, and ability to be easily moulded, sanded, and shaped. It also has a glossy finish that is compatible with many paints and glues.
Another way to identify plastic without a code is to perform a float test. Polyolefins, which include High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE), and Polypropylene (PP), generally float in water, while other types of plastics, such as Polyethylene Terephthalate (PET), generally sink. For example, a piece of a milk bottle made from HDPE will float, while a piece from a cool drink bottle made from PET will sink.
It is also possible to identify some plastics by their smell when burned. For example, PET smells similar to burnt sugar, PVC has an acrid smell like chlorine, LDPE and HDPE smell like candle wax, and Polypropylene smells similar to candle wax with a hint of paraffin. ABS plastic has a faint rubbery smell similar to styrene. However, it is advised to avoid burning plastics whenever possible due to the potential health risks associated with the inhalation of toxic fumes.
Additionally, some plastics can be identified by their physical properties and applications. For example, PP, or polypropylene, is often used for food packaging such as chocolate and chip wrappers. It feels firm and stiff, and unlike some other plastics, it does not stretch but tears easily.
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Feel: PP feels firm and stiff, and will tear without stretching
One way to identify plastic without a code is by its feel. PP, or polypropylene, has a particular feel that can help distinguish it from other plastics.
PP feels firm and stiff to the touch. This is in contrast to other plastics like LDPE, which is thin enough to crumble in your hands, or HDPE, which is tougher to deform and has a waxier appearance. PP's stiffness and firmness lend it a high resistance to fatigue. This means that it can take a lot of strain and deformation without cracking.
Another distinctive feature of PP is its lack of stretchiness. Unlike other plastics, PP does not stretch. When pulled, it simply tears and rips without any give. This is a key characteristic that helps identify PP by feel.
PP is commonly used for food packaging, such as chocolate and chip wrappers, or clear packets for clothing. It is also used for plastic lunch boxes, water cups, and food packaging boxes.
While feeling the plastic can provide clues as to its identity, it is not always a definitive method. Appearance, float tests, burn tests, and even smell can also be used in conjunction with the feel to help identify a plastic without a code.
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Frequently asked questions
The easiest way to distinguish between broad groups of plastic is by checking whether they float or sink. Polyolefins, which include High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE), and Polypropylene (PP), generally float in water, while other types of plastics tend to sink. You can also identify plastics by their smell when burned, but this is not recommended due to the potential release of toxic chemicals.
Some common types of plastic include Polyethylene Terephthalate (PET), Polyvinyl Chloride (PVC), Polypropylene (PP), Polystyrene (PS), and Polycarbonate (PC). PET is the most well-known member of the polyester family of plastic polymers and is often used in beverage containers. PVC is known for its resistance to moisture and is used in pipes and electrical cable insulation. PP is commonly used in food packaging and has a stiff and non-stretch texture. PS is a hard and brittle plastic used in disposable cutlery and CD cases. PC is a strong and impact-resistant plastic used in bulletproof glass and eyeglasses.
Many plastic products will have information about the type of plastic on the label. Look for abbreviations like "ABS," "PC," "PA," or "PP." Additionally, you can identify some plastics by their unique characteristics, such as the crinkling sound of LDPE bags or the stiffness of PP packaging.
