Maya Ellison
Plastic is a synthetic or semi-synthetic material that can be molded into solid objects. It is used in a multitude of products, from paper clips to spacecraft. However, not all plastics are created equal when it comes to their environmental impact, and some are more easily recycled than others. Knowing how to identify different types of plastic can help consumers make better-informed decisions about the products they buy and enable more effective recycling. The Society of the Plastics Industry introduced the Resin Identification Code (RIC) system, which categorizes plastic resins into seven groups. Each type of plastic has distinct properties, such as texture, appearance, density, and behaviour when exposed to heat or flame. These characteristics can be used to identify the type of plastic and its potential applications or environmental implications.
| Characteristics | Values |
|---|---|
| Texture | PE feels smooth, soft, and tough, with obvious marks after being scratched. MDPE and HDPE are hard and not easily breakable. PP feels smooth and is bendable without leaving marks after being scratched. PVC feels soft and tough and feels sticky. |
| Appearance | PE is milky white, translucent, and waxy when uncolored. MDPE and HDPE share the same characteristics. PP is white, waxy, and translucent when uncolored. PVC is yellowish, translucent, and glossy. ABS is milky white or beige, amorphous, opaque, and dull. |
| Sound | PE and LDPE produce a low sound when beaten. ABS produces a crisp sound when beaten. |
| Taste and Smell | PE, MDPE, HDPE, and ABS are tasteless and odorless. LDPE and HDPE smell like candle wax. Polypropylene smells like candle wax with a hint of paraffin. PET smells like burnt sugar. PVC has an acrid smell like chlorine. |
| Density | Different types of plastics have different densities, which can be used to identify them based on whether they float or sink in water. |
| Flammability | Different types of plastics burn differently and produce different types of smoke. |
| Recyclability | PET is the most widely recycled plastic in the world. HDPE is one of the easiest plastic polymers to recycle and is accepted at most recycling centers. PVC cannot be recycled through pyrolysis as it damages machinery. |
| Biodegradability | Thermodegradable plastics degrade when heat is applied. Biodegradable plastics are biologically degraded, e.g., by microbes. Photodegradable plastics degrade in sunlight. |
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What You'll Learn
Appearance: Smoothness, colour, transparency, glossiness, etc
The appearance of plastic can vary depending on its type, and this can be a simple way to identify it. PE, for example, feels smooth, soft, and tough, is extensible and bendable, and shows obvious marks when scratched. MDPE and HDPE, on the other hand, are hard and possess great rigidity and toughness. They appear milky white, translucent, and waxy when uncolored and float in water. PP, another type of plastic, also feels smooth but does not show marks when scratched. It is bendable and possesses great tensile strength and rigidity. When uncolored, it appears white, waxy, and translucent.
PVC, or polyvinyl chloride, is soft, tough, and sometimes sticky. It is yellowish, translucent, and glossy, with higher transparency than other plastics. It is also one of the most hazardous kinds of plastic. Polystyrene, also known as Styrofoam, is highly inflammable and dangerous as it can leach harmful chemicals when heated. It is not biodegradable and is often mistaken by animals for food.
PC, or polycarbonate, has a metallic feel and is hard, tough, and resistant to bending or impact. It appears as a white, light yellow, or amber crystalline powder. As a transparent solid, it is nearly colorless, tasteless, and odorless. ABS plastic feels hard, tough, rigid, and is not easy to break. Its appearance is milky white or beige, amorphous, opaque, and dull.
It is important to note that burning plastic can lead to environmental pollution and cause respiratory problems due to the release of poisonous chemicals. Additionally, while the float test and burn test can help rule out certain plastics, they may not always provide specific identification.
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Density: Floating or sinking in water
Density, or gravity dressing, is a simple method to identify different types of plastics. Different plastics have different densities, so leveraging differences in density can help identify plastics based on whether they float or sink in water.
The floating method can be used to separate PP and PE from PET. PP, PE, PS, PA, and ABS can be identified using saturated brine, while PP, PE, PS, PA, ABS, and PC can be identified with a saturated aqueous solution of calcium chloride. However, this method cannot separate PVC from PET because their densities are similar.
The float test is a good way to rule out certain plastics, but it is challenging to identify a specific type of plastic using this method alone. It is often used in conjunction with other methods, such as the burn test, to accurately identify plastic types.
The burn test involves holding a plastic sample to a flame and observing its ignition potential, the nature of the smoke, and whether the sample drips. After extinguishing the flame, one can cautiously smell the fumes to gain further clues about the plastic's identity. For example, PET smells like burnt sugar, PVC smells like chlorine, LDPE and HDPE smell like candle wax, and polypropylene has a scent similar to candle wax but with a paraffin element.
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Texture: Hardness, softness, stickiness, etc
Texture is a key differentiator between different types of plastics. Each plastic has its own unique characteristics, including hardness, softness, stickiness, and flexibility.
Polyvinyl Chloride (PVC), for instance, can feel soft and tough, and sometimes even sticky to the touch. It is also highly flexible and can be made softer by mixing it with other substances. This flexibility allows it to be used in plumbing, wiring, electrical cable insulation, and flooring. In contrast, Polyethylene Terephthalate (PET) is a much more rigid form of plastic. It is hard and strong, making it ideal for food and drink packaging as it prevents oxygen from entering and spoiling the product inside.
Polymethyl Methacrylate (PMMA) is another example of a hard plastic. It is transparent, resembling clear glass, yet it is not easily shattered. It can be bent when heated, showcasing its flexibility.
Low-Density Polyethylene (LDPE) is a thin, flexible form of plastic. It is so thin that it can crumble in your hands and make a loud, crackling noise. On the other hand, High-Density Polyethylene (HDPE) is tougher to deform and has a waxier appearance.
Polypropylene is another flexible plastic that can be bent but will not break easily. Its resistance to breakage makes it ideal for product containers.
Polystyrene is a plastic that stands out due to its brittleness. It can easily break without much force, and it is also highly flammable.
In addition to these examples, it is worth noting that the density of plastics can also be used to differentiate between types. The floating method, which involves placing plastics in water to observe their buoyancy, can indicate their density.
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Burning: Smoke, ignition, smell, melting, etc
Burning plastic to identify its type should only be done as a tentative test, as some complex compounds require rigorous analysis. It can be harmful to health and the environment and should be carried out with caution.
Firstly, heat the sample to around 500°F/260°C. If the material is a thermoplastic, it will soften at this temperature, whereas a thermoset will not. Next, hold the sample to a flame. If it ignites, observe the nature of the smoke, the presence of soot, and whether the sample drips. A blue flame with yellow tips indicates polyethylene, while a yellow flame with green tips indicates PVC. Polyvinyl chloride (PVC) produces a cloud of black smoke. Polyethylene and polypropylene are highly flammable and will continue to burn even after the flame is removed. Polystyrene is also highly flammable and produces a strong smell of burnt styrene.
After burning, extinguish the flame and cautiously smell the fumes. Comparison with a known sample can be helpful here. Finally, check your observations against the known characteristics of each plastic. An additional test can then be carried out to confirm the results.
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Recycling: Thermoset or thermoplastic
The majority of plastic that is manufactured is classified as a thermoplastic (80%) which means that theoretically, it can be reused. Thermoplastics can be easily recycled since they can be melted and reshaped into new products. Thermoplastics can go through the pyrolysis process to recycle the material, which involves heating the material at 400+ degrees with no oxygen to break it down into its original components.
However, this ease of recycling does not always translate into practice, and some types of plastic are responsible for more environmental damage than others. For instance, polycarbonates (PC) are used to build strong, tough products like eye protection, lenses for sunglasses, sports and safety goggles, mobile phones, and compact discs. In recent years, the use of these resins has been controversial due to their leaching at high temperatures, which releases bisphenol A (BPA), a potential environmental hazard that does not easily decompose in landfills.
On the other hand, thermoset plastics retain their shape once they have been moulded, making them difficult to recycle. To determine whether a material is a thermoset or thermoplastic, one can heat a stirring rod to about 500°F/260°C; if the material is a thermoplastic, it will melt. One can also apply a flame to a piece of plastic and observe its smoke and ignition potential. Different types of plastic produce distinct smells when burned; for example, PET smells like burnt sugar, PVC has an acrid chlorine odour, LDPE and HDPE smell like candle wax, and polypropylene has a paraffin-like scent.
Knowing how to identify the different types of plastic is important for consumers as it enables them to avoid certain types of plastics and recycle them more effectively. The Society of the Plastics Industry introduced the Resin Identification Code (RIC) system, which categorises plastic resins into seven groups to facilitate recycling. Codes 1, 2, and 5 are easy to recycle, and plastics that belong to group 1, like PET, are widely recycled and used for food and drink packaging.
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