Maya Ellison
There are several ways to identify plastic. The float test and burn test are two common methods for tentatively identifying plastics. The float test involves placing the plastic in water to see if it floats or sinks, which can indicate the type of plastic. The burn test involves heating a sample until it ignites and observing characteristics such as the nature of the smoke, the presence of soot, and the smell of the fumes. For a more advanced identification method, infrared analysis can be used to distinguish between different plastics.
| Characteristics | Values |
|---|---|
| Float Test | ABS sinks in water, Polypropylene floats |
| Burn Test | Observe nature of smoke, presence of soot, dripping, and smell the fumes |
| Solvent Test | Acetone, Methyl Ethyl Ketone (MEK), 2-Butanone, and Methylene Chloride dissolve ABS but not Polypropylene |
| Flammability | Observe if the plastic catches fire |
| Infrared Analysis | Measure the material with Infrared light or ATR (Attenuated total reflectance) |
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What You'll Learn
The float test
Preparing the Test:
- Gather your materials: All you need for this test is a container of water and the object you want to test. It is important to use a container that is tall and narrow enough to allow for a meaningful comparison between the water level and the object's buoyancy.
- Understand specific gravity: Specific gravity is the density of an object relative to water. An object with a specific gravity greater than 1 will sink, while an object with a specific gravity less than 1 will float. Keep in mind that some plastics have a similar specific gravity to water, so they may not float or sink but rather hover in the middle.
Performing the Float Test:
- Fill the container with water: Fill the container with enough water to completely submerge the object you are testing.
- Place the object in the water: Gently place the object in the water without applying force.
- Observe the object's behaviour: Pay close attention to whether the object sinks or floats. If it sinks, it is likely a thermoset plastic or a plastic with a specific gravity greater than 1, such as ABS (Acrylonitrile Butadiene Styrene). If it floats, it is likely a thermoplastic or a plastic with a lower specific gravity, such as polypropylene.
- Consider the presence of bubbles: Note that the presence of bubbles or voids in the plastic object can affect the results. Bubbles can increase the buoyancy of ABS, for example, making it float instead of sink.
Additional Considerations:
It is important to remember that the float test is not always definitive, especially when distinguishing between similar plastics. In such cases, additional tests, like the burn test or solvent test, may be necessary to confirm the type of plastic. Additionally, infrared analysis or Attenuated Total Reflectance (ATR) can provide more advanced identification methods.
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The burn test
Burning plastic can release pollutants like microplastics, bisphenols, and phthalates, which are toxins that can significantly impact neurodevelopment, endocrine, and reproductive functions. Therefore, it is important to exercise caution when performing a burn test.
Once the sample is burning, carefully extinguish the flame and pay attention to the fumes. Comparing the odor to that of a known sample can be helpful for identification. Finally, refer to the known characteristics of different plastics to make a tentative identification. It is generally recommended to perform an additional test to confirm the results.
While the burn test can provide tentative identification of plastic materials, it is important to note that some complex plastic compounds may require more rigorous analysis. Additionally, always take the necessary precautions when performing any test involving burning to ensure safety and minimize the potential impact on air quality and health.
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Check chemical resistance
Checking the chemical resistance of a plastic item is important to understand how long it will last and what it is suitable for. Chemical resistance charts are a great resource for determining which plastic should be used in a chemical environment. These charts show how different plastics interact with a variety of chemicals and other fluids across the pH scale, from milk to mineral oil to fruit juice.
The charts can indicate how well a plastic resists chemicals at different concentrations and will detail the extent of any chemical attack, environmental stress cracking, and physical absorption. For example, a slight attack by absorption may cause some swelling and a small reduction in mechanical properties, whereas a moderate attack will result in appreciable absorption and the material will have a limited life.
It is also important to consider how long a plastic item will be exposed to chemicals. Even chemical-resistant plastics can break down over time with prolonged exposure. Therefore, it is recommended to always "over-engineer" and choose a plastic that can more than handle your specifications.
Other environmental factors that can affect chemical resistance include temperature, concentration of the driving forces, duration, and mechanical load.
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Observe discolouration
Discolouration is a common issue with plastic objects, especially those that are old or have been exposed to sunlight. The process of plastic fading is complex and can be influenced by various factors, including the type of plastic, the presence of pigments, and the environment in which it is stored.
One of the most common signs of plastic discolouration is a yellow hue on the surface. This is often observed in plastic storage containers and can be caused by food particles that have been left for too long. To restore the container's original colour, one can use household products such as vinegar, rubbing alcohol, or denture-cleaning tablets. Leaving the container in direct sunlight for several hours can also help fade the discolouration.
In some cases, plastic discolouration can be more severe and may require more intensive treatments. For example, historical plastic objects from the 1950s to 1970s may exhibit inhomogeneous colours due to the degradation of certain pigments over time. This can be observed through microscopic analysis, which reveals the progressive loss of colour in pigment particles. Accelerated aging tests and colour metric measures are also used to study and simulate the discolouration process in historical plastics.
Discolouration can also occur during the manufacturing process of plastic objects. This may be due to factors such as temperature changes, exposure times, and the presence of certain stabilizers or ingredients in the plastic formulation. For instance, slight variations in temperature and exposure times can affect light-coloured nylons, and copper halide heat stabilizers have been known to cause discolouration in light colours. Additionally, the use of older equipment without proper decompression or control procedures can contribute to discolouration issues during manufacturing.
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Analyse the odour
Odour analysis is a critical aspect of identifying plastics and understanding their composition. Here are some key insights on analysing the odour of plastic materials:
Plastic Odours and Their Sources
Plastics are typically made of artificial polymers derived from oil, and these polymers themselves are odourless. However, the smell associated with plastics originates from small molecules that are volatile at room temperature. These molecules can be additives, leftover monomers from the polymerisation process, or breakdown products. For example, non-rigid PVC contains noticeable odours from plasticisers like phthalates.
Tangy Smells in Cheap Plastic
Cheap plastic items, particularly those from China, often emit a strong tangy odour. This smell is attributed to the presence of esters or other odour-masking substances. The intensity of this odour can vary, but it tends to linger and impregnate other objects, proving challenging to remove.
Odour Analysis Techniques
Odour analysis of plastics can be performed using standardised methods such as ISO 16000-28 and VDA 270. These methods assess both sensory and molecular characteristics. Fourier Transform Infrared (FTIR) spectroscopy is another technique used for plastic identification by analysing the object's characteristics and grouping similar plastics together.
Environmental and Safety Concerns
The emission of odours from plastics can be a concern in confined spaces like vehicles, buildings, and in products like toys and food packaging. It is essential to identify and manage these odours to maintain air quality and address potential health risks.
Identifying Plastics Through Odour
Plastics can often be identified by their specific odour, especially when they start to degrade. The Museum of Design in Plastics (MoDiP) has developed a flowchart to help identify plastics based on characteristics like smell, specific traits, and branding. This tool assists in quick identification, but technical analysis is still necessary for unambiguous confirmation.
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Frequently asked questions
There are a few ways to tentatively identify plastic. Firstly, the float test can be used to rule out certain plastics. Secondly, the burn test is a common method of identification, although it is destructive. Heat the object and note if it melts or burns. If it burns, observe the smoke, the presence of soot, and whether the sample drips. Then extinguish the flame and smell the fumes. Compare these observations with the known characteristics of different plastics.
Heat a stirring rod to about 500°F/260°C. If the material melts, it is a thermoplastic. If it doesn't melt, it is probably a thermoset.
Put a small drop of Acetone, Methyl Ethyl Ketone (MEK), 2-Butanone, or Methylene Chloride on the surface and smear it with a knife. If the plastic softens and dilutes, it is ABS. Another method is to rub the plastic with a paper towel and see if any discolouration occurs. ABS will usually show some dulling of the surface.
Infrared analysis can be used to identify different plastics. If the material is too thick for a transmission measurement, Attenuated Total Reflectance (ATR) can be used to measure inside the solid.
Some complex plastic compounds require a rigorous analysis for identification. These tests should only be used for tentative identification. It is usually desirable to perform additional tests to confirm the results.
