Chloe Zhang
Identifying the type of hard plastic can be done in several ways. One common method is to look for recycling symbols, usually found on the bottom or side of plastic products, indicating the resin identification code (RIC). Another approach is to distinguish plastics by their density, as some float in water while others sink. Burning plastics can also help identify them, but this method should be approached with caution due to the potential release of harmful fumes. Additionally, the appearance and feel of plastics can provide clues about their type, such as the hardness, rigidity, and colour. Some plastics may also have markings or labels that directly indicate the type of plastic used.
| Characteristics | Values |
|---|---|
| Appearance | Milky white, translucent, waxy, glossy, etc. |
| Feel | Smooth, soft, tough, sticky, etc. |
| Density | Float test, buoyancy |
| Burning | Smell, colour of smoke |
| Recycling symbols | Numbers 1-7, arrows forming a triangle |
| Markings | Angle brackets, e.g. |
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What You'll Learn
- Look for international codes: three arrows forming a triangle with a number in the middle
- Float test: put plastic in water to test buoyancy
- Burn test: different plastics have distinct burning characteristics
- Scratch test: LDPE/PP scratch easily; PET/HDPE don't
- Density: some plastics are harder and denser than others
Look for international codes: three arrows forming a triangle with a number in the middle
The symbols with numbers in them enclosed by three arrows forming a triangle are Resin Identification Codes. They are commonly known as "chasing arrows" symbols. These symbols are seen on everything from plastic bottles to recycle bins. However, the presence of the chasing arrows symbol on a product does not necessarily mean that it is made from recycled plastic or that it will be recycled if it is thrown into your recycle bin. It simply identifies the type of plastic resin used in the product.
The first two codes are the most commonly accepted by local recycle centers. The first code is 1: PETE, or polyethylene terephthalate, which is clear, tough, and has good gas and moisture barrier properties. It is often found in soft drink and water bottles, containers for salad dressing, vegetable oil, and peanut butter. The second most commonly accepted code is 2: HDPE, or high-density polyethylene, which is generally translucent and does not allow light to penetrate. Its symbol is often found on milk containers, household cleaning products, shampoo bottles, and motor oil bottles.
Other number symbols are also recyclable, but they may not be accepted by most curbside services or recycling kiosks. For example, the number 6 code is used for expanded polystyrene, which is a plastic resin typically composed of 95% air, making it very light. However, it is not a desirable resin to recycle and is thus not accepted for recycling in most communities.
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Float test: put plastic in water to test buoyancy
One way to identify the type of hard plastic is to perform a float test. This involves putting the plastic in water and observing its buoyancy. While this method can help rule out certain types of plastics, it may not always provide a definitive identification.
Prepare the Plastic Sample: Start by cutting or breaking off a small piece of the plastic you want to identify. Ensure that the sample is relatively uniform in size and shape to facilitate accurate testing.
Choose a Container: Select a container that is large enough to accommodate the plastic sample and filled with water. The container should be made of a material that will not react with the plastic, such as glass or stainless steel.
Fill the Container: Fill the chosen container with water, ensuring it is deep enough for the plastic sample to be fully submerged. The water should be at room temperature to avoid any thermal effects on the plastic.
Perform the Test: Gently place the plastic sample on the surface of the water. Observe whether the plastic sinks or floats. If it floats, note the level of buoyancy. Some plastics may float high on the water's surface, while others may float just below the surface.
Compare with Known Standards: Compare the buoyancy and floating behavior of your plastic sample with known standards or reference materials. Different types of plastics have varying densities, which affect their buoyancy. For example, polyolefins, such as polyethylene and polypropylene, are known for their low density and high buoyancy.
Analyze Results: Based on how the plastic sample interacts with the water, you can narrow down the possibilities of the plastic type. For instance, if the plastic sample sinks, it likely has a higher density than water. This could indicate the presence of certain plastics like polyvinyl chloride (PVC) or polystyrene, which are denser than water.
While the float test is a simple and relatively safe method of plastic identification, it should be combined with other tests, such as the burn test, for more accurate results. Additionally, always exercise caution when handling plastic, especially if you plan to burn it, as some plastics can release toxic fumes when ignited.
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Burn test: different plastics have distinct burning characteristics
Burning plastic releases harmful compounds into the atmosphere, and yet, plastics are often burned in open fires. One of the ways to identify the type of plastic is through a burn test.
The burn test is a preliminary identification technique that is not entirely reliable on its own. Plastics with additives like flame retardants behave differently when burned, and composite materials that include multiple plastics or fillers exhibit blended burn behaviours that can be hard to identify. However, when used together with advanced techniques like infrared spectroscopy or pyrolysis-gas chromatography, the burn test can help achieve accurate material identification.
The burn test involves using a flame source, such as a lighter, torch, or bottled-gas burner. The plastic is then subjected to the flame, and the following characteristics are observed:
- Odour: The smell produced when burning plastic can help identify the specific type of polymer used. For example, polyolefins smell like candles, PVC smells like chlorine, ABS has a sweet smell, styrene is not as sweet, nylon smells like burning hair, and epoxies smell like a burnt-out electric motor.
- Material Drips: Whether the plastic drips during combustion can also help identify the type of plastic. Thermoplastics, for instance, drip during combustion because they become pliable when exposed to heat, whereas thermosets maintain their form and do not drip when exposed to flame.
- Smoke Characteristics: The type and amount of smoke emitted can also provide clues about the type of plastic.
In addition to the burn test, the float test has also been used to identify plastics. However, these tests are better suited for ruling out certain plastics rather than specifically identifying them.
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Scratch test: LDPE/PP scratch easily; PET/HDPE don't
One way to identify the type of hard plastic is by performing a scratch test. LDPE (Low-Density Polyethylene) and PP (Polypropylene) are relatively softer plastics and can be scratched easily. On the other hand, PET (Polyethylene Terephthalate) and HDPE (High-Density Polyethylene) are harder plastics and are more scratch-resistant.
LDPE is a flexible polyethylene resin used in various applications, such as food packaging and pharmaceutical packaging. It has excellent resistance to acids and bases. PP is also used in food and beverage packaging, automotive parts, and consumer products. It is lightweight, chemically resistant, and fatigue-resistant. However, both LDPE and PP are softer plastics compared to PET and HDPE, making them more prone to scratches.
PET, on the other hand, is a thermoplastic polymer resin known for its excellent mechanical, thermal, and chemical resistance properties. It is widely used in clothing fibres, disposable beverage bottles, and engineering resins. PET has superior thermal resistance compared to PP and other polyethylenes.
HDPE is a stronger and more durable type of polyethylene with a higher density than LDPE. It is commonly used in packaging, construction materials, and automotive components. HDPE has excellent impact strength, chemical resistance, and a high strength-to-weight ratio. Due to its higher density and linear molecular structure, it exhibits higher rigidity and impact resistance than LDPE and PP. This makes HDPE more resistant to scratches.
By performing a simple scratch test, you can quickly identify whether the hard plastic is LDPE, PP, PET, or HDPE. This can be useful for various purposes, such as choosing the appropriate adhesive for repairs or understanding the recycling properties of different plastics.
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Density: some plastics are harder and denser than others
The density of plastics varies, with some being harder and denser than others. This density difference leads to varying sinking and floating behaviours in water and other solutions. For instance, low-density polyethylene can remain above water, while polystyrene products float. In contrast, polyvinyl chloride, polypropylene, and high-density polyethylene are denser and sink in water.
The density of plastics is essential for classifying products and their containment, as specific applications require plastics with particular weight characteristics. For example, high-density polyethylene (HDPE) is strong and resistant to acidic chemicals, making it suitable for containers like bleach bottles, fuel tanks, and motor oil storage. On the other hand, low-density polyethylene (LDPE) is flexible and can change shape without damage, making it useful for applications that require pliability.
The appearance and texture of plastics can also indicate their density. LDPE, for instance, is thin enough to crumble in one's hands, and it produces a crackling noise. In contrast, HDPE is tougher to deform and often has a translucent, milky white coating, although it appears waxier when uncoloured. Polypropylene, a bendable material, is another example of a plastic that resists breakage due to its fatigue resistance.
The float test is a simple method for testing the density of plastics by observing their buoyancy in water. However, it is important to note that this method may not always accurately identify a specific type of plastic. Additionally, the combustion method can be used to identify plastics based on their combustion characteristics, such as ease of burning, odour, flame characteristics, and changes in the state of the plastic during burning.
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Frequently asked questions
Hard plastics can be identified through various methods, including:
- Locating three arrows forming a triangle with a specific number in the middle. Number 1, for example, pertains to polyester and PET plastics, whereas number 2 covers High-Density Polyethylene.
- Looking at the recycling symbols to find the Resin Identification Code (RIC), which is a number from 1 to 7 that corresponds to a specific type of plastic resin.
- The floating method, which tests the weight of plastics by putting them in water and witnessing their buoyancy.
- Burning, which can help rule out certain plastics, although it may not always be safe or accurate.
- Appearance and feel, such as the fact that PE feels smooth and soft, while PVC feels sticky.
Some common hard plastics include:
- Polyethylene terephthalate (PET or PETE), which is clear, strong, lightweight, and widely used for beverage bottles and food containers.
- High-density polyethylene (HDPE), which is stiff, durable, and opaque, commonly used for milk jugs, detergent bottles, and toys.
- Polyvinyl chloride (PVC), which feels soft and tough, and is one of the most hazardous types of plastic.
- ABS, which feels hard, tough, rigid, and is commonly used for home appliance casings and mechanical parts.
Plastic resin codes, or RICs, are numbers from 1 to 7 that correspond to specific types of plastic resins. Here are some additional codes beyond the ones mentioned earlier:
- 3: Polyvinyl chloride (PVC or V), which is flexible, durable, and widely used for pipes, wire insulation, and flooring.
- 4: Low-density polyethylene (LDPE), which is flexible, translucent, and used for plastic bags, film, and tubing.
- 5: Polypropylene (PP), which is strong, rigid, and heat-resistant, commonly used for containers, packaging, and automotive parts.
- 6: Polystyrene (PS), which is rigid, brittle, and used for disposable cups, containers, and packaging.
- 7: Other, which includes all other types of plastics that don't fit into the previous categories, such as polycarbonate (PC) and acrylic (PMMA).
When identifying plastics, it is important to take the following safety precautions:
- Avoid burning plastics as it can release harmful fumes and chemicals. If you must perform a burn test, do it in a well-ventilated area.
- Always wear gloves and eye protection when handling plastics to avoid exposure to potentially harmful substances.
- Be cautious when using the floating method, as some plastics may contain chemicals that can contaminate water.
- Consult a professional or a reliable guide if you are unsure about the type of plastic you are dealing with.
