Maya Ellison
Plastic is everywhere, from water bottles to clothing, and it's essential to recycle it for the well-being of our planet. However, with various plastic types, some reusable and some hazardous, it's crucial to know how to distinguish between them. The easiest method is to look for the international plastic identification code: three arrows forming a triangle with a number in the middle. Additionally, you can identify plastics by their density using the floating method. Different plastics also have distinct reactions to heat, changing in appearance and leaving behind unique smells. For instance, PVC has an acrid, chlorine-like smell, while LDPE and HDPE smell like candle wax.
| 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 |
| Float test | Polyolefins float, nylon sinks |
| Burn test | Observe the nature of the smoke, presence of soot and dripping |
| Smell | PET smells like burnt sugar, PVC smells like chlorine, LDPE and HDPE smell like candle wax, Polypropylene smells like candle wax with paraffin, ABS and polystyrene smell like styrene with ABS also having a faint rubbery smell |
| Flame colour | 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 |
| Density | Polyethylene, polypropylene, polyvinyl chloride, and polystyrene products are less dense than water |
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What You'll Learn
Using the floating method to test density
The density of plastics varies, leading to different sinking and floating behaviours in water and other solutions. The floating method can be used to test the density of plastics by observing their buoyancy when placed in water. If the density of the plastic is greater than that of water, it will sink, and if it is less dense, it will float. This method can help identify different types of plastics, as their densities vary. For example, low-density polyethylene (LDPE) and polystyrene products will float, while PVC and polystyrene with a density of 1.05 to 1.07 will sink.
To perform the floating method test, you can follow these steps:
- Prepare a container of water at room temperature. Temperature changes affect the density of water, so using room temperature water ensures consistent results.
- Gather samples of the plastics you want to test. Ensure that the samples are small enough to float or sink freely in the water.
- Place each plastic sample into the water one at a time.
- Observe whether the plastic floats or sinks. You can use letters or other markings to indicate which samples float and which sink.
- Compare your results with known densities of plastics. For example, you can find density information for common plastics like LDPE, high-density polyethylene (HDPE), polyvinyl chloride (PVC), and polypropylene (PP).
- Use the buoyancy and density information to identify the type of plastic. For instance, if a sample floats and you know it has a density less than that of water, you can narrow down the possibilities based on the known densities of different plastics.
It is important to note that the floating method is a simple way to test the density of plastics, but it may not always be accurate, especially for modified plastics or those with additives that change their density. Additionally, this method only provides information about density, while other identification techniques, such as burn tests, chemical solubility assessments, and recycling symbols, can provide a more comprehensive understanding of plastic types.
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Identifying international plastic identification codes
Plastic identification is important for understanding the basics of plastics and what each type represents. There are several methods to identify plastics, including the float test, burn test, and locating the international plastic identification codes.
The Resin Identification Code (RIC) is a set of symbols that appear on plastic products to identify the plastic resin the product is made of. Developed in 1988 by the Society of the Plastics Industry (now the Plastics Industry Association) in the United States, the RIC was originally a set of three arrows forming a triangle with a specific number in the middle. These international codes offer clarity on plastic identification based on chemical makeup and recycling grading. Symbol 1, for example, represents polyester and PET plastics, which are common in clothing, plastic bottles, and insulation materials. Since 2008, the RIC has been managed by ASTM International, which replaced the arrows with a solid triangle, although the old symbols are still commonly used.
The presence of a recycling code, chasing arrows logo, or resin code does not indicate that a material is recyclable; it explains what the item is made of. Various countries have adopted different recycling codes, with some having more detailed systems than others. For instance, China's polymer identification system has seven plastic classifications, five post-consumer path symbols, and 140 identification codes. The Sustainable Packaging Coalition in the United States has also created a "How2Recycle" label to indicate whether a plastic product is widely recycled, which aligns with how consumers use the RIC.
In addition to the RIC, plastic identification can be based on the appearance and feel of the plastic, as well as its density. The classification of plastic products depends on their weight, which can be tested using the floating method by observing buoyancy in water. Different plastics also experience unique chemical reactions when interacting with heat, resulting in changes in appearance and smell.
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Testing appearance, feel and smell
While it is difficult to identify plastic types with certainty without a professional analysis, there are some ways to differentiate plastics by testing their appearance, feel, and smell.
Testing Appearance
One way to identify different types of plastics is by locating three arrows forming a triangle with a specific number in the middle. These are international codes that offer clarity on plastic identification based on chemical makeup and recycling grading. For example, the number 1 symbolises polyester and PET plastics, which are common household items such as clothing, plastic bottles, and insulation materials. Number 2 symbolises High-Density Polyethylene (HDPE), which is strong and resistant to acidic chemicals.
Testing Feel
The float test and burn test are traditional methods for ruling out certain plastics, although they may not always lead to specific identification. Different types of plastics experience distinct chemical reactions when exposed to heat. For example, polyethylene and polypropylene are flammable plastics that continue burning even after removing the fire source. Polyvinyl, on the other hand, is nonflammable and does not sustain the flame. Polystyrene is highly flammable and produces a lingering burnt styrene smell, along with a cloud of black smoke.
Testing Smell
Plastics in everyday use are typically made of artificial polymers derived from oil. While the polymers themselves are odourless, the smell comes from small molecules that are volatile at room temperature, such as additives, leftover monomers, or breakdown products. For example, water bottles made of PET (polyethylene terephthalate) left in the sun can have a distinct taste due to contamination from the plastic's starting materials, such as bisphenol A. Some plastics may also have a tangy odour, which could be due to additives like cheap orange flavouring.
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Burning and observing smoke, soot and dripping
Burning a piece of plastic and observing its smoke, soot, and dripping behaviour can help identify its type. This is because different plastics exhibit distinct characteristics when burned. However, it is important to conduct these tests in a controlled and well-ventilated environment to ensure safety and clear results. Here are the key factors to consider:
Smoke
The type and amount of smoke produced during combustion can be indicative of the plastic type. For example, polystyrene (PS) and polyvinyl chloride (PVC) typically generate thick black smoke. On the other hand, Polyacetal (POM) tends to produce clean residue, while Polypropylene (PP) burns without leaving much residue.
Soot
The presence of soot indicates incomplete burning, which frequently occurs in plastics with high carbon content and additives. Soot is easily identifiable by its black, powdery appearance. For instance, polystyrene produces a sooty flame, whereas polypropylene burns with a clean, blue flame.
Dripping
Thermoplastics tend to drip during combustion because they become pliable when heated. This is in contrast to thermosets, which maintain their form and do not drip when exposed to flame. Therefore, observing whether the plastic drips during burning can help distinguish between thermoplastics and thermosets.
Flame Colour
Observing the colour of the flame can also provide valuable information about the plastic type. For example, polypropylene burns with a stable blue flame, while polyethylene gives off a yellow flame. Polyvinyl chloride produces a yellow-green flame, and polystyrene burns with a sooty flame.
Odour
Each plastic type releases unique odours when burned, which can be used for identification. For instance, polyvinyl chloride emits a sharp, acrid smell, similar to burning diesel. In contrast, polyethylene releases a sweet and gentle odour, while polypropylene has a candle-like or paraffin-like scent.
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Checking chemical structure and compliance with laws
Plastics are classified by their chemical structure, which determines their physical properties and behaviour. The most common classification is based on the chemical structure of the polymer backbone and side chains. The structure of these side chains influences the properties of the polymer, such as plasticity, allowing them to be moulded, extruded, or pressed into various solid forms.
Plastics can be classified by the chemical process used in their synthesis, such as condensation, polyaddition, and cross-linking. They can also be classified by their physical properties, including hardness, density, tensile strength, thermal resistance, and glass transition temperature. For instance, the floating method can be used to test the density of plastics by putting them in water and observing their buoyancy.
Plastics can also be classified by their resistance and reactions to various substances and processes, such as exposure to organic solvents, oxidation, and ionizing radiation. For example, different types of plastics experience a unique chemical reaction when exposed to heat, changing their appearance and smell.
When it comes to compliance with laws and regulations, plastic products are subject to restrictions on the use of certain chemical substances and mixtures. For instance, the EU has a permanent ban on the use of phthalates in toys, and the US government has banned certain types of phthalates commonly used in plastics. Manufacturers and importers of plastic products must conduct lab testing to ensure their products comply with relevant standards and do not contain restricted substances above the allowed limits.
Additionally, laws and regulations are encouraging companies to manufacture less-toxic plastics and promote the use of recyclable and biodegradable materials. For example, the city of Berkeley in California requires compostable packaging for takeaway food and reusable containers for eat-in establishments.
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Frequently asked questions
The easiest way to identify plastic is by locating three arrows forming a triangle with a specific number in the middle. These international codes offer clarity to plastic identification based on chemical makeup and the grading for recycling. Symbol 1 is for polyester and PET plastics, which are common in households. Symbol 2 covers High-Density Polyethylene, which is strong and resistant to acidic chemicals.
The floating method can test the weight or density of various plastics by putting them in water and witnessing their buoyancy. Depending on whether they sink or swim will tell you which types of plastic have a higher density than others.
If you must, a small whiff of the smoke from burning plastic will give you further clues as to its identification code. PET smells similar to burnt sugar, while PVC has an acrid smell like chlorine. LDPE and HDPE smell like candle wax, while Polypropylene smells similar to candle wax but with an element of paraffin.
To initially determine whether a material is thermoset or thermoplastic, heat a stirring rod to about 500°F/260°C. If the material is a thermoplastic, it will melt. If not, it is probably a thermoset.
