Chloe Zhang
Plastic is everywhere, from beverage bottles to mechanical parts. However, not all plastics are created equal, and their environmental impact varies. To make informed decisions about the products we buy, it's essential to understand the different types of plastics and how to identify them. The Society of the Plastics Industry introduced the Resin Identification Code (RIC) system, categorising plastic resins into seven groups. Each type of plastic has distinct properties, structures, and sizes, largely determined by the monomers used in their creation. Identification methods include appearance, burning behaviour, and density. For example, PE feels smooth, soft, and tough, while PVC feels sticky and soft, and polyolefins float, unlike nylon, which sinks.
| Characteristics | Values |
|---|---|
| Appearance | PE feels smooth, soft and tough, is extensible and bendable, with obvious marks after being scratched. MDPE and HDPE are hard, and possess great rigidity and toughness. The appearance is milky white, translucent and waxy when uncolored. PP feels smooth, without marks after being scratched. It is bendable, not easy to break, and possesses great tensile strength and rigidity. When uncolored, the appearance is white, waxy and translucent. PVC feels soft, and the product is tough and feels sticky. The appearance is yellowish, translucent and glossy. ABS feels hard. It is tough, hard, rigid and not easy to break. The appearance is milky white or beige, amorphous, opaque and dull. Polystyrene can be solid or foamed. |
| Floatation | PE, MDPE, HDPE, and PP float in water. ABS sinks in water. Polystyrene floats on water. |
| Burning | A blue flame with a yellow tip indicates polyolefins and nylon. A yellow flame with a green tip indicates PVC. A yellow flame with dark smoke could be PET or polycarbonate. |
| Thermoplastics vs. Thermosets | Heat a sample to 500°F/260°C; if it melts, it is a thermoplastic; if not, it is probably a thermoset. |
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What You'll Learn
Appearance: PE is bendable, MDPE/HDPE are hard, PVC is sticky, etc
The appearance of plastic can be a simple way to identify the type. PE, or polyethylene, feels smooth, soft, and tough, and is bendable. It also shows obvious marks when scratched. It is also one of the only plastics that can float in water. MDPE and HDPE, on the other hand, are hard and possess great rigidity and toughness. They are also resistant to drops and shocks. The appearance of these plastics is often milky white, translucent, and waxy when uncoloured. They make a low sound when beaten. Common products made from these plastics include mulch, handbags, water pipes, oil barrels, and beverage bottles.
PVC is also soft and tough, but it has a sticky feel. It is yellowish, translucent, and glossy. It is also flexible and commonly used for soles, toys, wire sheaths, doors, and windows.
Another way to identify plastics is through a combustion test. This involves burning the plastic and observing characteristics such as the smell, flame colour, and state after burning. For example, PET burns and emits black smoke and has a sour smell. PP keeps burning and has a yellow and blue flame, smelling like a candle.
A third method is to identify plastics by their density. The buoyancy test involves placing the plastic in water and observing whether it floats or sinks. The density of common plastics is between 0.9 and 2.0 g/cm³, and only PP and PE have densities less than 1.0, so they will float.
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Burn test: Observe smoke, soot, dripping, and smell
Conducting a burn test can help identify different types of plastic. However, it is important to note that this test should only be done in a controlled and safe environment. The test involves observing the smoke, soot, dripping, and smell of the burning plastic. Firstly, hold a sample of the plastic to the edge of a flame until it ignites. If no flame is produced immediately, hold it in the flame for about 10 seconds. Once ignited, note the nature of the smoke and the presence of soot in the air. Also, observe if the plastic drips while burning.
Extinguish the flame and cautiously smell the fumes. It is helpful to have a known sample for comparison when identifying the odour. Different types of plastics exhibit distinct characteristics when burned. For example, polypropylene (PP) burns easily, with melting and dripping. The flame colour is yellow on top and blue below. After removing the flame, it continues to burn, emitting a petroleum odour. On the other hand, polyethylene terephthalate (PET) ignites easily, softens, and forms bubbles with an orange flame. It produces a small amount of black smoke. Once the flame is removed, it slowly extinguishes, leaving an acidic smell.
Polytetrafluoroethylene (PTFE) does not burn but instead decomposes under intense heat, releasing a pungent odour of hydrofluoric acid. Polymethyl methacrylate (PMMA) burns and melts, forming bubbles. The flame is light blue, and the material appears white. Interestingly, PMMA burns without smoke but continues to burn after the flame is removed, emitting a strong fruity or decaying vegetable odour. Polyethylene (PE) catches fire easily, accompanied by melting and dripping. Similar to PP, the flame colour is yellow on top and blue below. It continues to burn after the flame is removed, releasing a wax-like burning smell.
The burn test is a helpful tool for identifying different types of plastics, ensuring proper disposal and recycling methods. However, it is important to remember that some complex plastic compounds may require more rigorous analysis for accurate identification.
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Float test: Some plastics float, others sink
The float test is a simple method for determining the type of plastic. Some plastics will float, while others will sink. This test is based on the density of the plastic; plastics with a density lower than that of water will float, while those with a higher density will sink. There are three plastics that float on water: HDPE, LDPE, and PP. The density of these plastics varies depending on their manufacturing process, but they generally have a lower density than water. On the other hand, plastics with a higher density, such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene), will sink. The density of PET, for example, is 1.38-1.39, which is higher than that of water, causing it to sink.
The float test is a useful way to narrow down the possibilities when trying to identify an unknown plastic. However, it should be noted that this test alone may not be sufficient to definitively identify the specific type of plastic. Other tests, such as the burn test, can be used in conjunction with the float test to help identify the plastic more accurately. Additionally, visual inspection and knowledge of the object's typical use can provide clues about the type of plastic. For instance, a milk jug is likely made of high-density polyethylene (HDPE), while a plastic container that held laundry soap is often made of the same material.
The ability to identify different types of plastic is important for recycling purposes. Plastics are identified by codes developed by the SPI (Society of Plastic Industries) in 1988. These codes, known as SPI codes or recycling codes, range from #1 to #7 and correspond to specific types of plastics. For example, #1 represents polyethylene terephthalate (PET or PETE), #2 is high-density polyethylene (HDPE), #3 is polyvinyl chloride (PVC), and so on. Understanding these codes helps in making informed decisions about plastic usage and contributes to proper recycling practices.
It is worth noting that the misconception of a floating "island of garbage" in the ocean is not entirely accurate. While there is indeed a massive accumulation of plastic in the ocean, it consists of both floating and sinking plastics. The plastics that sink are presumed to be resting at the bottom of the oceans, contributing to the environmental impact of plastic pollution. This highlights the importance of proper waste management and recycling practices to mitigate the negative effects of plastic on the planet and human health.
In conclusion, the float test is a simple and practical method for distinguishing between floating and sinking plastics. By understanding the density characteristics of different plastics, we can narrow down the possibilities when identifying unknown plastics. However, combining this test with other identification methods, such as burn tests and knowledge of plastic codes, will provide a more comprehensive understanding of the specific type of plastic. Proper plastic identification plays a crucial role in recycling, environmental conservation, and promoting the well-being of our planet and its inhabitants.
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Flame colour: Polyolefins and nylon produce a blue flame
One way to determine different types of plastics is by observing the colour of the flame they produce when burned. Burning polyolefins and nylon, for instance, produces a blue flame with a yellow tip. Polyolefins are a family of polyethylene and polypropylene thermoplastics, which are produced mainly from oil and natural gas. They are one of the most popular plastics in use today due to their versatility.
The float test is another way to distinguish polyolefins from nylon. Polyolefins generally float in water, while nylon (PA) sinks.
It is important to note that the float test and burn test are not always accurate in specifically identifying a plastic. They are, however, useful in ruling out certain plastics. When conducting the burn test, be cautious as molten plastic can drip and cause burns.
Additionally, the smell of the smoke produced during burning can provide further clues about the type of plastic. For example, PET, which is a type of plastic that sinks, smells similar to burnt sugar, while PVC, which produces a yellow flame with a green tip, has an acrid smell like chlorine. LDPE and HDPE smell like candle wax, and polypropylene has a similar scent but with a hint of paraffin.
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Chemical makeup: Each plastic has a unique chemical composition
Plastic is an indispensable component in manufacturing a wide array of products, from water bottles to automotive parts. Each type of plastic has a unique chemical composition, which serves as a distinguishing factor.
The production of plastics starts with the distillation of crude oil, which separates it into lighter components called fractions. These fractions are mixtures of hydrocarbon chains with varying molecular sizes and structures. One of the crucial compounds for plastic production is naphtha, which is created during this process. Two key processes are involved in creating plastics: polymerisation and polycondensation. Both methods require specific catalysts to link monomers (ethylene and propylene) into long polymer chains. The resulting polymer chains exhibit distinct characteristics, such as specific properties, structures, and sizes, depending on the types of monomers used.
Thermoplastics, a common type of plastic, exhibit unique properties where they soften upon heating and harden upon cooling. The float test and burn test are traditional methods for ruling out certain plastics, but they may not always provide specific identifications.
To identify plastics, it is essential to understand their chemical makeup and SPI codes. For instance, polyolefins and nylon can be differentiated by their response to the float test, with polyolefins floating and nylon sinking, despite sharing similar flame characteristics. Additionally, PVC (polyvinyl chloride) exhibits a yellow flame with a green tip upon contact, while PET or polycarbonate produces a yellow flame with dark smoke.
The chemical composition of plastics significantly influences their physical and mechanical properties, impacting their suitability for specific applications. By understanding the unique chemical makeup of each plastic type, we can make more informed decisions about their usage and recycling, contributing to the well-being of our planet.
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