Ethan Clarke
There are several ways to identify plastics. One of the most common methods is by locating the three arrows that form a triangle with a number in the middle, known as the resin identification code. These international codes are based on the chemical makeup and recycling grading of plastics. For example, Symbol 1 represents polyester and PET plastics, which are commonly found in clothing, plastic bottles, and insulation materials. Another way to identify plastics is through their appearance and texture. For instance, polyethylene feels smooth and flexible, while low-density polyethylene is thin enough to crumble in your hands. Additionally, the float test and burn test are traditional methods used to rule out certain plastics, but they may not always provide specific identification.
| Characteristics | Values |
|---|---|
| International codes | Three arrows forming a triangle with a specific number in the middle |
| Symbol 1 | Polyester and PET plastics |
| Symbol 2 | High-Density Polyethylene |
| Symbol 3 | PVC items |
| Symbol 7 | Acrylic, nylon, and polycarbonate |
| Appearance | Smooth, flexible, crinkly, waxy, translucent, etc. |
| Feel | Hard, soft, crumple, flexible, etc. |
| Sound | Loud, soft, swishing, crisp, etc. |
| Float test | Polyolefins float in water |
| Burn test | Heating to 500°F/260°C to determine if thermoplastic or thermoset |
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What You'll Learn
- Burn test: Heat to 500°F/260°C to differentiate between thermoplastics and thermosets
- Float test: Observe buoyancy to determine density
- Observe appearance: Look for transparency, colour, and glossiness
- Feel the texture: Identify smoothness, stickiness, and flexibility
- Check for symbols: Look for the resin identification code, usually surrounded by chasing arrows
Burn test: Heat to 500°F/260°C to differentiate between thermoplastics and thermosets
The burn test is a preliminary technique to identify plastics and differentiate between thermoplastics and thermosets. It involves heating a metal or glass rod to around 500°F (260°C) and pressing it against the plastic sample. If the material softens, it is likely a thermoplastic, whereas if it remains unchanged, it is probably a thermoset. Thermoplastics tend to soften or melt when heated, while thermosets maintain their solid form.
To perform the burn test, hold the sample near the edge of a flame, such as from a cigarette lighter or gas burner. If it does not ignite immediately, hold it in the flame for no more than 10 seconds. Observe the colour of the flame, the nature of the smoke, and whether the sample drips while burning. A blue flame indicates thorough combustion with fewer impurities, while yellow or orange flames suggest the presence of carbon or other contaminants. The density and colour of the smoke, as well as the presence of soot, can also help identify the specific type of plastic.
After extinguishing the flame, cautiously smell the fumes, preferably with a known sample for comparison. Finally, refer to the known characteristics of each plastic type to make a tentative identification. While the burn test provides initial information, it may not be conclusive for complex plastic compounds. More advanced methods, such as spectral analysis, infrared spectroscopy, or pyrolysis-gas chromatography, may be necessary for accurate identification.
It is important to note that the presence of additives, such as flame retardants, can modify how materials behave when burned, making identification more challenging. Composite materials that include multiple plastics or fillers may exhibit blended burn behaviours, requiring detailed examination for accurate identification.
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Float test: Observe buoyancy to determine density
The float test is a simple method for determining the density of an object relative to water. This test can be performed by placing an object in water and observing whether it sinks or floats. If the object floats, it is less dense than the water; if it sinks, it is denser.
This test can be used to compare the densities of two objects of similar weight, such as a wax candle and a piece of clay. In this example, the candle will float, and the clay will sink, despite the candle weighing more. This outcome occurs because the candle is less dense than the clay relative to the density of water.
The float test can also be used to determine the density of an object by gradually breaking off pieces from a larger mass and observing at what point the smaller pieces begin to float. For example, a large ball of clay will sink, but if it is broken in half, the smaller ball may float. This process can be repeated to create smaller and smaller balls of clay until they are no longer dense enough to overcome the surface tension of the water and sink.
While the float test is a useful tool for understanding relative density, it is important to note that density is not the only factor influencing whether an object sinks or floats. The shape and surface area of an object can also play a role in its buoyancy. Additionally, the float test may not be suitable for identifying very small objects, as they may be affected by surface tension rather than buoyancy forces.
In summary, the float test is a straightforward method for observing buoyancy and determining relative density. By placing objects in water and observing their behaviour, we can learn about their density compared to water and make predictions about their likelihood of sinking or floating.
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Observe appearance: Look for transparency, colour, and glossiness
One of the ways to identify plastic is by observing its appearance, specifically its transparency, colour, and glossiness. Transparency is an important factor in identifying the type of plastic. For example, High-Density Polyethylene (HDPE), which is used in shopping bags, has a translucent and milky white coating, while Low-Density Polyethylene (LDPE) is mostly colourless and transparent.
Colour can also be a distinguishing factor. Uncoloured HDPE, for instance, has a waxier appearance than LDPE. Polypropylene (PP), used in food packaging, is known for its firm and stiff characteristics, and it usually comes in a variety of colours.
The glossiness or texture of plastic can provide clues as well. Polyethylene has a smooth texture and is flexible but can leave marks. LDPE is thin and can crumble easily, producing a loud crackling noise. On the other hand, HDPE feels harder and crinklier. PP has a bendable flexibility but will not easily break.
In addition to these visual cues, the sound produced when plastic is crumpled can be a distinctive identifier. LDPE produces a soft, swishing sound, like green leaves blowing in the wind, while HDPE emits a crisper, crinklier noise, reminiscent of dry leaves being squished.
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Feel the texture: Identify smoothness, stickiness, and flexibility
One of the easiest ways to identify plastic is by feeling its texture. This can give you clues about the type of plastic you're dealing with. For example, polyethylene has a smooth texture and is flexible. On the other hand, Low-Density Polyethylene (LDPE) is thin and can even crumble in your hands, making a crackling sound. High-Density Polyethylene (HDPE), in contrast, is tougher to deform and often has a waxy, translucent, and milky white appearance.
Polypropylene is another type of plastic that is characterised by its flexibility and resistance to fatigue. It can be bent but will not break easily, making it a reliable choice for product containers. Polyvinyl Chloride (PVC), on the other hand, can feel soft, tough, and sometimes sticky. It is also known for its higher transparency compared to other plastics, as well as its glossy texture and yellowish appearance.
Polystyrene is a type of plastic that stands out due to its brittleness. It can easily break without much force, making it quite distinct from the other plastics mentioned. Additionally, when burned, polystyrene produces a strong smell similar to burnt styrene, which lingers even after the fire is extinguished.
The float test is another method that can be used to identify the density of plastics. By placing a piece of plastic in water and observing whether it sinks or floats, you can determine its relative density. However, this method may not always provide a definitive identification of the specific type of plastic.
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Check for symbols: Look for the resin identification code, usually surrounded by chasing arrows
One of the easiest ways to identify plastic is by checking for symbols, specifically the resin identification code. This code is usually surrounded by chasing arrows forming a triangle, with a specific number in the middle. These international codes are based on the chemical makeup and recycling grading of plastics, providing clarity for identification and proper recycling practices.
The resin identification codes are as follows:
- Symbol 1 represents polyester and PET plastics. This category includes common household items such as plastic bottles, clothing, and insulation materials.
- Symbol 2 corresponds to High-Density Polyethylene (HDPE), which is known for its strength and resistance to acidic chemicals. Bleach bottles, fuel tanks, and motor oil storage containers often bear this symbol.
- Symbol 3 is assigned to PVC items, which can be both rigid and flexible. This category encompasses door and window frames, sporting balls, water pipes, and electrical wire insulation.
- Symbol 7 represents a catch-all category for various plastic types, including acrylic, nylon, and polycarbonate. These plastics are often non-recyclable and are combined with other polymers to create a wide range of products.
It is important to note that these symbols are not the only means of identification, as some plastics may not have visible markings. Additionally, while the float test and burn test have been traditionally used to rule out certain plastics, they may not always provide specific identification. Therefore, a combination of identification methods, including visual inspection, sensory evaluation, and knowledge of plastic characteristics, can be employed for accurate identification.
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Frequently asked questions
There are a few simple ways to identify plastic, including:
- Looking out for a number, known as the "resin identification code", surrounded by a triangle of arrows.
- Feeling the plastic — polyethylene, for example, feels flexible and smooth to the touch.
- The float test — this can determine the weight and buoyancy of the plastic.
The seven resin identification codes are numbers 1-7. Each number corresponds to a specific type of plastic:
- Number 1: Polyester and PET plastics.
- Number 2: High-Density Polyethylene (HDPE).
- Number 3: PVC.
- Number 7: Acrylic, nylon, and polycarbonate.
PET (or Polyethylene terephthalate) is a member of the polyester family of plastic polymers. It is commonly used for packaging foods and liquids such as water and soda.
Thermoplastic becomes mouldable at about 500°F/260°C, whereas thermoset does not.
