Chloe Zhang
Plastics are everywhere, from water bottles to car bumpers, and with so many different types, it can be tricky to tell them apart. There are several ways to identify plastics, from checking the manufacturer's label to performing a burn test. The SPI resin identification coding system, for example, uses a triangle of three arrows with a number in the middle to indicate the type of plastic. Another method is to test the density of the plastic by seeing if it floats or sinks in water. A more extreme method is the burn test, which involves holding a small piece of plastic to a flame and observing its reaction. However, this method should be used with caution due to the toxicity of plastic combustion products.
| Characteristics | Values |
|---|---|
| Burn test | Whether the material will freely burn in air when a flame is removed |
| Density test | Whether the plastic floats or sinks in water |
| Beilstein copper wire test | Whether the plastic contains chlorine |
| Acetone test | Whether the plastic becomes "tacky" on exposure |
| Stress test | Whether the plastic turns white when bent |
| Welding test | Whether the plastic melts |
| International recycling codes | A triangle of three arrows with a number in the middle |
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What You'll Learn
Burn test
The burn test is a preliminary method to identify plastics by correlating a plastic sample's composition with a set of observable properties. It involves placing a flame against a plastic sample and monitoring its reaction. This test is useful for differentiating between thermoplastics and thermosets.
When performing a burn test, it is important to take safety precautions such as wearing protective gear and conducting the test in a well-ventilated area due to the potential release of harmful fumes or chemicals.
- Nature of Smoke: The density and colour of the smoke produced during burning can help identify the type of plastic. For example, PVC emits a sharp, acrid smell, while polyethylene releases a sweet and gentle odour.
- Soot Presence: Soot formation during combustion indicates incomplete burning, which is common in plastics with high carbon content and additives.
- Material Dripping: Thermoplastics tend to drip when exposed to heat due to their pliable nature, while thermosets maintain their form and do not drip.
- Flame Behaviour: Observe whether the material freely burns in air when the flame is removed.
- Residue: After the flame is extinguished, carefully smell the fumes as each plastic produces a unique odour.
While the burn test provides initial information, it may not be conclusive on its own. Advanced techniques, such as spectral analysis, infrared spectroscopy, or pyrolysis-gas chromatography, are necessary for accurate identification of complex polymers and composite materials.
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Density
There are a variety of methods for testing the density of plastics. One of the most popular non-destructive solutions is the use of Density Gradient Columns, which offer extremely accurate density measurements. They use at least two miscible liquids to build a density gradient, and calibrated glass floats are used to calibrate the column. The test material is then added, and once equilibrium is reached, the density of the material can be found.
Another method is the use of an Apparent Bulk Density Apparatus Ray-Ran, which is a fast and reliable instrument for measuring the apparent density of moulding materials. It offers a measuring method for determining the apparent density of moulding materials that can be poured through a specified funnel, compliant with ISO Method R60.
A simple test that can be carried out at home to identify different plastics is the float test. This involves cutting a small piece of plastic and dropping it into a glass of water. If it sinks, it is likely to be HDPE or PVC, and if it floats, it is likely to be PET or LDPE. This test is based on the principle that different plastics have different densities, which affect whether they float or sink.
It is important to note that while density testing can provide valuable information about plastics, it may not always be sufficient to fully identify a specific type of plastic. Other tests, such as burn tests and scratch tests, can be used in conjunction with density tests to more accurately identify different types of plastics.
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International recycling codes
Recycling codes are used to identify the materials out of which an item is made, to facilitate an easier recycling process. The presence of a recycling code, a chasing arrows logo, or a resin code, is not an indicator that a material is recyclable; it explains what the item is made of.
The Resin Identification Code (RIC) is a technical standard with a set of symbols appearing on plastic products that identify the plastic resin out of which the product is made. It was developed in 1988 by the Society of the Plastics Industry (now the Plastics Industry Association) in the United States. Since 2008, it has been administered by ASTM International, an international standards organisation. The RIC symbols are often mistaken for the recycling symbol, due to their resemblance. The old RIC symbols featured arrows in a triangle, similar to the universal recycling symbol, but these have been replaced by a solid triangle to address consumer confusion.
The SPI resin identification coding system has been widely adopted by plastics manufacturers to label their products for post-consumer recycling. The SPI stated that one purpose of the original SPI code was to " [provide] a consistent national system to facilitate recycling of post-consumer plastics."
Some countries have a finer-grained system with more recycling codes. For example, China's polymer identification system has seven different classifications of plastic, five different symbols for post-consumer paths, and 140 identification codes.
The "How2Recycle" labels encourage consumers to check with local facilities to see what plastics each municipal recycling facility can accept.
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Susceptibility to acetone
The susceptibility of plastics to acetone varies widely. While acetone has no impact on some materials, it can cause others to soften, smear, or even dissolve. The chemical makeup of plastics plays a crucial role in their susceptibility to acetone.
Some plastics, such as polycarbonate and polysulfone, share a similar chemical structure with acetone, making them particularly vulnerable. These plastics may soften or dissolve when exposed to acetone. On the other hand, plastics like FEP and PCTFE exhibit excellent resistance, maintaining their shape and strength even under prolonged exposure.
Acetal, a plastic often used in labs and workshops, falls into the "fair" category for acetone resistance. While it can withstand occasional contact, continuous exposure can lead to a loss in durability over time. Therefore, it is not recommended for long-term projects where acetone is regularly used. Polyethylene and Ullem are also worth considering, as they offer good resistance with only minor changes over time.
To completely avoid the effects of acetone, certain plastics should be chosen. FEP, PFA, ETFE, and PCTFE are known to stand strong against acetone, keeping their structural integrity and mechanical strength intact. Additionally, PTFE (Teflon) is known for its exceptional chemical resistance, including resistance to acetone, and is available in transparent sheets.
When selecting plastics for specific applications, it is essential to consult a plastic chemical resistance chart. This chart will help identify the most suitable plastic for the intended use, ensuring the product's longevity and performance.
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Reaction under stress
One way to tell different plastics apart under stress is to observe their reaction to bending. For example, PVC whitens under stress when bent, whereas PET does not.
Another method is to perform a burn test, which correlates a plastic sample’s composition with a set of observable properties. For instance, PVC produces an acrid smell like chlorine when burned, while LDPE and HDPE smell like candle wax. Polypropylene smells similar to candle wax but with an element of paraffin. ABS and polystyrene smell like styrene, with ABS also having a faint rubbery smell. Polyolefins, which generally float and have the same flame and dripping "firebomb" effect, ignite quite readily and should be tested with caution. A blue flame with a yellow tip indicates polyolefins and nylon, with the latter sinking in water. A yellow flame with a green tip indicates PVC, while a yellow flame with dark smoke could be either PET or polycarbonate. Finally, a yellow flame with sooty, dark smoke could indicate polystyrene or ABS.
It is important to note that the float test and burn test are not always reliable for specifically identifying a plastic, but they can be useful for ruling out certain types. Additionally, the toxicity of plastic combustion products should be considered, and these tests may not be suitable for all samples.
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Frequently asked questions
There are several methods to identify different types of plastics. The first is to look for a label or international code, which is usually indicated by three arrows forming a triangle with a specific number in the middle. Symbol 1, for example, represents polyester and PET plastics. If there is no label, you can test the density of the plastic by dropping it in a small vial of liquid with a known density.
You can test the density of plastics by dropping a sample of the plastic in a small vial of liquid with a known density. If the plastic sample floats, it is less dense than the liquid, and if it sinks, it is more dense.
Other methods to identify plastics include the burn test, the Beilstein copper wire test, and the acetone test. The burn test involves bringing a small piece of plastic in contact with a flame and observing its reaction. The Beilstein test indicates the presence of chlorine, for example, in PVC. Most plastics, except polyethylene and polypropylene, will become tacky when exposed to acetone.
It is important to identify the type of plastic before welding, as you can only weld "like" materials. Most plastics nowadays are stamped with identification codes, but in the absence of these codes, a flame test can be performed. This involves using a pair of tweezers to ignite a small sliver of the plastic over a Bunsen flame.
Some common plastic identification symbols include:
- Symbol 1: Polyester and PET plastics
- Symbol 2: High-Density Polyethylene (HDPE)
- LDPE: Low-Density Polyethylene, which is more flexible than HDPE and often found in plastic bottles
- Vinyl or Polyvinyl Chloride (PVC): Often used in plumbing and can be flexible depending on the amount of plasticizer used
