Omar Khalid
There are many different types of plastics, and identifying them can be tricky. Some common methods for identification include the float test, the burn test, and feeling the material. The float test involves placing the plastic in water and observing its buoyancy. The burn test involves holding the plastic to a flame and observing the colour of the flame, the nature of the smoke and the presence of soot. Feeling the material can also provide clues about the type of plastic, as some plastics have distinct textures and levels of flexibility. Other methods include observing the appearance of the plastic, such as its colour or transparency, and checking for markings or labels that indicate the type of plastic.
| Characteristics | Values |
|---|---|
| Appearance | Polyethylene is flexible and smooth. LDPE is thin and can crumble in the hand, making a loud noise. HDPE is tougher to deform and has a milky white coating. Polypropylene is bendable but won't break easily. Polystyrene is brittle and can break without much force. |
| Sound | LDPE makes a soft, swishing sound. HDPE makes a crisper, crinklier sound. PP or polypropylene sounds similar to HDPE. |
| Buoyancy | Polyethylenes, polypropylene, and polyvinyl chloride have high density. LDPE, polystyrene, and polyvinyl have low density. |
| Flammability | Polyethylene and polypropylene are flammable and continue to burn when the fire is out. Polyvinyl is nonflammable. Polystyrene is incredibly flammable. |
| Burning Appearance | Polyethylene burns with a blue flame and yellow tips. PVC plastic burns with yellow flames and green tips. |
| Burning Smell | Polyolefins smell like candles. PVC smells like chlorine. ABS has a sweet smell. Styrene isn't as sweet. Nylon smells like burning hair. Epoxies smell like a burnt-out electric motor. |
| Reaction to Heat | Thermoplastics soften when heated to 500°F/260°C. |
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The float test
There are three types of plastics that commonly float on water: HDPE (High-Density Polyethylene), LDPE (Low-Density Polyethylene), and PP (Polypropylene). These plastics are often used in a variety of products, such as food packaging and plastic bottles. For example, HDPE is commonly used for plastic shopping bags, while LDPE is known for its flexibility and can be found in plastic wrap. PP, on the other hand, is bendable but resistant to breakage, making it ideal for product containers.
To perform the float test, simply fill a container with water and gently place the plastic item into the water. If the plastic floats, it is likely one of the three floating plastics mentioned above. However, it is important to note that the float test may not always provide a definitive identification of the plastic type. For example, ABS (Acrylonitrile Butadiene Styrene), which is a type of plastic that typically sinks, can sometimes float due to the presence of bubbles or voids in the material.
Additionally, the float test can be used to reinforce the identification of other plastics that sink. For instance, PET (Polyethylene Terephthalate), PVC (Polyvinyl Chloride), and PS (Polystyrene) are denser than water and will sink in the float test. This test can be a useful initial step in identifying the type of plastic, but it is often combined with other tests, such as the burn test or a density test, for more accurate results.
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The burn test
To perform the burn test, start by holding a flame source, such as a lighter, torch, or bottled gas burner, against a small piece of plastic. Observe the colour of the flame, the nature of the flame, whether there is any soot, and if the plastic drips. Then, blow out the flame and fan the smoke towards your nose to discern its smell. Each plastic material releases a unique odour when burned. For instance, PVC emits a sharp, acrid smell, whereas polyethylene yields a sweet and gentle aroma.
The density and colour of the smoke produced during the burn test are also indicators of the plastic type. Incomplete burning, signified by the presence of soot, suggests that the material remains partially unoxidized. Additionally, thermoplastics tend to drip during combustion because they become pliable when heated, whereas thermosets maintain their form and do not drip.
It is important to note that the burn test should be conducted with caution. Always ensure a safe environment and take the necessary precautions when working with open flames and combustible materials.
While the burn test provides initial insights, it is not a definitive method for plastic identification. More advanced techniques are required to accurately identify complex and composite materials.
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Appearance and texture
One of the easiest ways to identify plastic is by its appearance and texture. The surface of the plastic can be a key indicator of its type. For example, Low-Density Polyethylene (LDPE) is thin and flexible, often used in plastic bags, while High-Density Polyethylene (HDPE) is tougher and more rigid, commonly found in hard containers like detergent bottles. Polypropylene is also flexible but stands out for its high resistance to fatigue, making it ideal for product containers as it can withstand strain without cracking. Polyvinyl Chloride (PVC) has a glossy texture and a range of colours, from clear to yellowish. It is known for its durability and softness, sometimes feeling sticky to the touch.
Another distinguishing factor is the transparency of the plastic. LDPE is often translucent, while HDPE usually appears as a milky white colour. PVC is known for its high transparency, and polystyrene is easily recognisable due to its brittleness and tendency to break easily. The appearance of plastics can also change when exposed to heat, and they may produce unique smells that can aid in identification. For example, PVC emits dangerous chlorine gases when heated, resulting in an acrid smell, while Polyethylene Terephthalate (PET) gives off an odour similar to burnt sugar or candy floss.
The float test is another method to distinguish plastics by their density. LDPE, HDPE, and polypropylene typically float in water, while other plastics like nylon will sink. The burning or melt test can also be used to identify plastics by observing their reaction to heat. LDPE and HDPE, for instance, tend to drip and produce a flame with a blue or yellow tip, while PVC softens instead of melting and emits chlorine fumes.
It is important to note that these visual and tactile methods are preliminary identification techniques. For complex plastic compounds, a rigorous analysis may be required for accurate identification. Additionally, it is recommended to avoid exposing yourself to plastic fumes whenever possible, as some plastics release toxic gases when burned.
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Flexibility
The flexibility of plastic is determined by its cellular structure, or how its molecules bond with each other, and how tightly they are formed. The stiffness of plastics is described by flexural modulus (the ability of a material to bend), which is measured in pounds per square inch (psi). The higher the flexural modulus, the stiffer the material; the lower the flexural modulus, the more flexible it is.
Polyethylene is one of the most widely used materials for manufacturing a range of products due to its flexibility and resistance. It is a polymer formed by a string of hydrogen and carbon atoms, repeatedly chained together. It is a thermoplastic obtained by the polymerisation of ethylene. Linear Low-Density Polyethylene (LLDPE) is the most flexible of the plastic sheeting films. It is blended with LDPE to make it more pliable and softer. LDPE is the most common type of plastic sheeting and is very flexible, conforming well to a variety of surfaces. However, it is not as strong, dense, or puncture-resistant as other types of plastic sheeting.
There are international codes that offer clarity on plastic identification based on chemical makeup and recycling grading. Symbol 5 plastics are flexible polypropylenes that can resist most acids and solvents. These include ropes, beverage bottles, plates, and dishwasher-proof food containers. In contrast, Symbol 6 is for polystyrene, which is stiff and rigid. This includes CD cases, ballpoint pens, and kitchen appliances and electronics casings.
Tests can be performed to determine the flexibility of plastics. One simple test is to cut a rectangular piece from a fixed place on the product and hang it horizontally by the border of a table, with half of its length hanging off the edge. The floating method is another test, where plastic is placed in water to observe its buoyancy. Low-density polyethylene, for example, can remain above water.
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Flammability
While it is possible to identify plastics by their flammability, it is not recommended due to the negative health and environmental impacts of burning plastic. If you choose to identify plastics in this way, you should be aware that different plastics produce different flame colours. For example, a blue flame with yellow tips indicates burning Polyethylene, while yellow flames with green tips indicate PVC plastic. Polyethylene and polypropylene are flammable plastics that continue to burn even after the fire is out. Over time, the material starts melting and releases the smell of burning wax. Polyvinyl, on the other hand, is non-flammable and does not keep the flame going. Polystyrene is another plastic that is highly flammable and produces a cloud of black smoke.
There are standardised testing procedures for determining the flammability characteristics of plastics. These tests involve subjecting specimens of the plastic material to a flame ignition source for a specified period. The tests can be conducted with the specimen in a horizontal or vertical position, and the flame may be applied once or multiple times, depending on the test. The flammability of a plastic is determined by its tendency to either extinguish or spread the flame once ignited. Some tests also measure the resistance of the plastic to ignition from electrical sources.
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Frequently asked questions
You can identify plastics through their appearance, feel, buoyancy, and smell. For example, LDPE is thin enough to crumble in your hands and makes a loud, soft, and swishing noise. On the other hand, HDPE is tougher and makes a crisper and crinklier sound. Polyethylene and polypropylene are flammable and continue to burn even after the fire is out, while polyvinyl is nonflammable.
The float test involves putting plastic in water to observe its buoyancy. Plastics with lower density, such as low-density polyethylene and polystyrene, will float, while others with higher density will sink.
Common plastic symbols include "PC" for polycarbonate, "PA6" for nylon, and "ABS" for acrylonitrile butadiene styrene. These symbols are often found within angle brackets on product labels or packaging.
Different plastics have distinct reactions to heat and fire. For example, polyethylene burns with a blue flame and yellow tips, while PVC plastic produces yellow flames with green tips. The smoke and soot produced, as well as the melting behaviour, can also help identify the type of plastic.
Burning plastics can release toxic fumes that are harmful to both human health and the environment. It is recommended to avoid using fire as a method for identifying plastics due to these negative effects.
