Sofia Martinez
Plastic is a polymeric material that can be moulded or shaped, usually by applying heat and pressure. It is made from oil, a carbon-rich raw material, and can be divided into two categories based on chemical composition. The first category is made up of polymers with only aliphatic (linear) carbon atoms in their backbone chains, while the second category, heterochain polymers, contain atoms such as oxygen, nitrogen or sulphur in their backbone chains. The chemical composition of plastics can be determined by various methods, including FTIR, DSC, mass spec, GC, and TGA, but this equipment can be costly. Another way to distinguish different types of plastics is by their density, using the floating method to test buoyancy. International codes in the form of three arrows forming a triangle with a number in the middle can also help identify plastic types based on chemical makeup and grading for recycling.
| Characteristics | Values |
|---|---|
| Plastic type | PET, PVC, Polypropylene, Polystyrene, Polycarbonate, Polyester, Polyethylene, Polyurethane |
| Chemical composition | Carbon, hydrogen, oxygen, nitrogen, sulfur, chlorine, ethylene, propylene, styrene, vinyl chloride, acrylonitrile |
| Physical properties | Rigid, flexible, transparent, tough, lightweight, flimsy, fragile |
| Uses | Food containers, bottles, toys, pipes, wires, cups, utensils, windows, doors, balls, bags, wraps, packaging, automotive parts, bottle caps, drinking straws, disposable utensils |
| Identification methods | International codes/symbols, density/weight, floating test, smell/taste, appearance, XRF, elemental analysis, FTIR, DSC, mass spec, GC, TGA |
| Hazards | Bisphenol A (BPA), additives, processing aids, starting substances, NIASs (impurities, reaction by-products), toxic compounds |
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What You'll Learn
- Check the plastic container for a symbol with three arrows forming a triangle and a number in the middle
- Identify the type of plastic by its density using the floating method
- Understand the chemical composition of plastics, e.g. hydrocarbons, polymers, and monomers
- Learn how to distinguish different types of plastics by their taste, smell, and feel
- Familiarize yourself with the common types of plastics, such as PET, PVC, and Polypropylene
Check the plastic container for a symbol with three arrows forming a triangle and a number in the middle
Check the bottom or sides of your plastic container for a symbol with three arrows forming a triangle with a number in the middle, known as the "chasing arrows" symbol. This symbol is often mistaken for a recycling symbol, but it actually indicates the type of plastic resin used to make the container. The number inside the triangle is a Resin Identification Code, which helps determine the appropriate management and recycling route for the plastic. For example, the number 6 inside the triangle indicates that the container is made of expanded polystyrene, which is typically not accepted for recycling due to its low density and high cost of handling and transportation.
It's important to note that the chasing arrows symbol does not guarantee that the item can be recycled. Different recycling programs accept different types of plastics, and the symbol can be misleading. If the number in the triangle is not 1 or 2, it may not be recyclable through your local program and could require specialised facilities. Check your local recycling guidelines to determine which types of plastics are accepted in your area.
While the chasing arrows symbol provides information about the type of plastic, it does not indicate whether the plastic is recyclable. The recyclability of plastic depends on various factors, including the specific type of plastic, the availability of specialised recycling facilities, and the guidelines of your local recycling program.
To further complicate matters, there are challenges in determining the exact chemical composition of plastics beyond broad categories like polyethylene and polystyrene. While there are tests that can provide a good estimate, such as combining FTIR with DSC or using mass spec and GC, they often require expensive equipment or laboratory services.
Therefore, when it comes to recycling plastic containers, it's important to consider not only the chasing arrows symbol but also the specific guidelines and capabilities of your local recycling program. Additionally, reducing plastic consumption and switching to more sustainable alternatives can be more effective in mitigating plastic pollution.
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Identify the type of plastic by its density using the floating method
The floating method is a simple way to identify the type of plastic by its density. This method involves placing a sample of the plastic in water and observing whether it floats or sinks. If the plastic sample has a lower density than water, it will float; if it has a higher density, it will sink. This method is often used in classroom experiments to teach students about the different densities of plastics.
To perform this test accurately, it is important to prepare the plastic samples properly. Samples should be cut into different shapes or labelled with a code to distinguish them. It is also crucial to test the samples in advance to ensure they float or sink as expected. The density of the water should be known, and temperature changes should be avoided as they can affect the density of the water.
When conducting the floating test, observe whether the plastic sample floats or sinks. If it floats, you can infer that the plastic has a lower density than water. For example, low-density polyethylene (LDPE) and polystyrene products tend to float in water due to their lower density.
On the other hand, if the plastic sample sinks, it indicates a higher density than water. Plastics with higher densities include PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene). These plastics are commonly used in products like drink bottles, food containers, and electrical wire insulation.
It is worth noting that the floating method is just one way to identify the type of plastic. Other methods include using international codes, such as the triangle with arrows and a number, which provides information about the chemical makeup and recyclability of the plastic. Additionally, some advanced equipment and techniques, such as FTIR, DSC, mass spec, and GC, can be used to determine the exact chemical composition of plastics, although these methods may be more costly.
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Understand the chemical composition of plastics, e.g. hydrocarbons, polymers, and monomers
Plastics are derived from hydrocarbons, which are molecules composed of hydrogen and carbon atoms. These molecules can be combined into long chains, known as polymers, which are the primary components of plastics. The process of creating these chains is called polymerization, and the individual molecules that form polymers are called monomers.
Monomers are the foundational building blocks of plastics. They are simple molecules that, when combined, form complex polymers. The discovery and understanding of monomers date back to the early 1800s, with the work of August Wilhelm von Hofmann, who first coined the term. Monomers can be categorized based on their chemical structure and source, and different types of monomers result in plastics with varying attributes. For example, the type of monomer used and the polymerization method can significantly alter a plastic's tensile strength, or its ability to stretch without breaking.
Some common types of monomers include ethylene and propylene, which are important for producing polyethylene (PE) and polypropylene (PP) plastics. The ethylene monomer can be altered by substituting one of its hydrogen atoms with chlorine, resulting in a material known as polychloroethene. Additionally, monomers can provide plastics with specific characteristics, such as chemical resistance and electrical insulation. For instance, certain monomers can give plastics the ability to resist degradation due to chemical exposure, making them suitable for use in harsh chemical environments.
Polymer identification testing can be performed to determine the chemical composition of plastics. While there is no single test to determine the exact composition, combining different laboratory techniques, such as FTIR with DSC, or adding a mass spec or TGA, can provide very accurate results. These tests are typically conducted by specialized analytical labs and can be costly.
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Learn how to distinguish different types of plastics by their taste, smell, and feel
Distinguishing different types of plastics based on their taste, smell, and feel is a challenging task that requires expertise. Here are some ways to help you identify them:
Taste
Tasting plastic is not recommended due to potential health risks. However, if you are referring to the taste left on the plastic, it can be challenging to identify specific plastics based on this alone.
Smell
Plastics can emit distinct odours, especially when burnt or degraded. Polyethylene and polypropylene, for instance, may produce a burning wax smell when burnt. Polyvinyl has a distinct chlorine odour when heated for an extended period. Polystyrene emits a lingering burnt styrene smell. Recycled plastics often have unpleasant odours from volatile organic compounds (VOCs).
Feel
The tactile sensation of plastics can vary significantly. Polyvinyl Chloride (PVC) and Polyethylene (PE) feel soft, while Polycarbonate (PC) has a metallic feel. PE is also smooth, extensible, and bendable, leaving obvious marks after being scratched. Low-Density Polyethylene (LDPE) is thin and can crumble in your hands, producing a loud crackling noise. High-Density Polyethylene (HDPE) is tougher and less flexible, with a milky white coating. Polypropylene (PP) is smooth, bendable, and resistant to breaking. Polystyrene (PS) feels smooth but is brittle and breaks easily.
Additionally, the density of plastics can be a distinguishing factor. The "floating method" involves placing plastics in water to observe their buoyancy. Polyvinyl Chloride, Polyethylene, and Polypropylene tend to sink, while Low-Density Polyethylene and Polystyrene can remain afloat.
It's important to note that the easiest way to identify plastics is by locating the recycling symbol: three arrows forming a triangle with a number in the middle. These symbols provide information about the chemical makeup and recycling guidelines for different types of plastics.
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Familiarize yourself with the common types of plastics, such as PET, PVC, and Polypropylene
Polyethylene terephthalate (PET) is a commonly used plastic that is known for its strength, lightweight nature, and transparency. It is widely used in the food and beverage industry, particularly for bottles and containers, due to its ability to be safe for storing food and its excellent transparency for packaging that requires clarity, such as water and soda bottles. PET is also recyclable and is considered environmentally friendly. However, it may contain antimony trioxide, a carcinogen that can be released at warm temperatures.
Polyvinyl chloride (PVC) is another prevalent plastic type known for its durability and versatility. It can be formulated as either rigid or flexible, making it suitable for various applications such as piping, flooring, cables, roofing materials, and medical devices. PVC is also fire-resistant, electrically insulating, and resistant to biological contamination, making it ideal for electrical and medical applications. However, PVC is challenging to recycle and has been associated with serious health and environmental concerns.
Polypropylene (PP) is a lightweight plastic known for its excellent resistance to chemicals and heat. It is commonly used in food packaging, medical devices, auto parts, and consumer goods. PP is also relatively environmentally friendly and recyclable. Its resistance to fatigue makes it ideal for living hinges in products like Tupperware and disposable diapers.
High-Density Polyethylene (HDPE) is a strong and thick plastic resin used for grocery bags, milk jugs, shampoo bottles, medicine bottles, and more. It is one of the easiest plastic polymers to recycle and is accepted at most recycling centers worldwide. However, some studies suggest that HDPE can leach estrogen-mimicking additive chemicals when exposed to ultraviolet light, potentially disrupting the human hormonal system.
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Frequently asked questions
The easiest way to identify the chemical makeup of a plastic container is by locating three arrows forming a triangle with a number in the middle. These international codes offer clarity to plastic identification based on chemical makeup and the grading for recycling.
These numbers range from 1 to 7, with each number representing a different type of plastic. For example, Symbol 1 pertains to polyester and PET plastics, while Symbol 2 covers High-Density Polyethylene.
PET is a polymer chain of carbon, hydrogen, and oxygen molecules. It is made up of repeating (C10H8O4) units (ten carbon, eight hydrogen, and four oxygen).
PVC is made up of ethylene (containing hydrogen and carbon) and chlorine. The monomers of PVC are made up of (C2H3Cl) chains (two carbon, three hydrogen, and one chlorine).
Plastics are made from oil, a carbon-rich raw material. Most plastic is chemically inert and will not react with other substances. However, some plastics contain toxic chemicals and are hazardous. Polycarbonate (PC), for example, is toxic due to traces of Bisphenol A (BPA) and is banned in several countries.
