Ethan Clarke
The density of a substance is independent of the size of the sample, which makes it a useful property for distinguishing between different types of plastics. A simple experiment to identify plastics by density involves placing samples of polymers found in everyday plastics into liquids of known density and observing whether they float or sink. This method is used in many commercial firms to separate plastics for recycling. For example, heavy plastics will sink in a brine solution, while lighter plastics float. The density of a plastic can also be calculated by weighing a cubic centimetre of each type of plastic and dividing the mass by the volume. The density of different plastics varies, and they exhibit distinct combustion characteristics, appearance characteristics, and recycling symbols, which can also help identify the type of plastic.
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What You'll Learn
Using the floatation method
The floatation method is a widely used technique for identifying plastics by density. This method involves placing samples of plastics in liquids of known densities and observing whether they float or sink. The plastics used in this experiment can be sourced from everyday items, such as plastic bottles, food packaging, and disposable containers. It is important to test the plastic samples in advance to ensure they exhibit the expected floatation behaviour.
To prepare for the experiment, different solutions with known densities are required. Examples of such solutions include DI water, NaCl, sodium bromide, NaI, and calcium chloride solutions, each with varying densities. For instance, DI water has a density of 1 g/cm3, while NaCl has a density of 1.2 g/cm^2. These solutions are then labelled with their respective densities, usually in g/cm^3 at a standard temperature of 20°C.
Next, the plastic samples should be prepared by cutting them into squares of approximately 4 x 4 mm. Each sample should be labelled with a unique identifier, such as letters A through G, to facilitate easy reference during the experiment. It is important to ensure that the solutions and samples are stored at room temperature to avoid variations in density due to temperature changes.
During the experiment, each plastic sample is placed into the different solutions, one at a time. The behaviour of the samples is then observed and recorded in a data table, indicating whether they float or sink in each solution. By comparing the floatation behaviour with the known densities of the solutions, students can identify the densities of the plastic samples. Additionally, they can refer to a table of known polymer densities to determine the specific type of plastic.
The floatation method is a simple and effective way to identify plastics by density. It allows for the separation of plastics based on their buoyancy, with lower-density plastics floating and higher-density plastics sinking. This technique has been applied in various fields, including waste management and plastic recycling, to segregate specific plastics, such as polystyrene (PS), polyethylene terephthalate (PET), and polyvinyl chloride (PVC), from mixed waste streams.
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Density calculations
To calculate the density of a plastic material, we use the formula: Density = Mass/Volume. This formula enables us to express the density in various units, such as grams per cubic centimeter (g/cm³), grams per cubic meter (g/m³), or kilograms per liter (kg/L). For example, if we have a sample of PVC (polyvinyl chloride) with a mass of 1.40 grams and a volume of 1 cubic centimeter, we can calculate its density as 1.40 g/cm³.
In practical experiments, students often test the density of plastics by placing samples in liquids of known densities. By observing whether the plastic samples float or sink, they can identify the density of the plastic. This method is commonly used in recycling processes, where different plastics are separated by floating them in brine solutions. Lighter plastics float, while heavier ones sink, allowing for effective separation.
Additionally, visual and tactile inspections can aid in identifying certain types of plastics. For instance, polypropylene (PP) is semi-transparent, waxy, and smooth to the touch, while polyethylene (PE) is a white powder or transparent resin that feels wax-like and shows scratch marks. Nylon (PA) has a milky white, gel-like appearance with a hard surface that feels warm to the touch.
Online composite density calculators are also available for convenience. These tools allow users to input the weight percentage of fillers and polymers to predict the final density of their plastic mixtures. By understanding the density of plastics and utilizing these calculation methods, we can improve the quality and performance of plastic products, optimize production processes, and facilitate effective recycling.
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Recycling symbols
- Symbol 1 with the acronym PETE PETG plastic or just PET: This plastic is used for drinks bottles and some food packaging and is widely recycled.
- Symbol 2 with the acronym HDPE: This is one of the most commonly used plastics in the United States and is used for cleaning product bottles, milk cartons, etc. It is widely recycled.
- Symbol 3 with the letter “V” or the acronym PVC: This plastic is used for car parts, window fittings, etc. It is not easily recyclable.
- Symbol 4 with the acronym LDPE: This plastic is used for plastic bags and wrapping and is not usually recycled but can be recycled at specialist points.
- Symbol 5 with the acronym PP: This plastic is used for some tubs and trays and is widely recycled.
- Symbol 6 with the acronym PS: This plastic is used for takeaway boxes, disposable cutlery, etc. It is not easily recyclable.
- Symbol 7 with the word “OTHER”: This category includes plastics like acrylic plastic, polycarbonate plastic, nylon, and fiberglass. Some of these plastics can be recycled at specialist points.
It is important to note that the "chasing arrows" symbol does not necessarily mean the product is recyclable. The number inside the triangle helps identify the type of plastic used for the product. By understanding the number classifications, we can determine the appropriate disposal route. Additionally, local recycling programs may vary, so it is essential to check with your local program to ensure proper disposal.
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Physical appearance
The density of plastics is an important property that affects their strength, rigidity, and other physical characteristics. Each type of plastic has distinct physical characteristics, including colour, lustre, transparency, and hardness. These characteristics can be identified through visual inspection and touch.
Polypropylene (PP), for example, is white or semi-transparent when uncoloured, with a waxy feel, and can be bent without breaking. It is also scratch-resistant, rigid, and has good tensile strength. Polyethylene (PE), on the other hand, is a white powder or transparent particulate resin that feels smooth and waxy. It is soft and stretchable but breaks easily when bent.
Polyethylene can be further classified into different forms, such as low-density polyethylene (LDPE) and high-density polyethylene (HDPE). LDPE is commonly used for packaging and bags and is known for its transparency and clarity. It has a lower density than HDPE, typically ranging from 0.91-0.93 g/cm3. HDPE, often used for pipes, tends to be opaque and can vary in colour from white to black. It has a higher density, usually between 0.94-0.97 g/cm3. The stretch test can be used to differentiate between these two types: LDPE is easily stretched and tough to break, while HDPE is challenging to stretch and will break before reaching its elastic limit.
Acrylonitrile Butadiene Styrene (ABS) is another plastic with distinct physical characteristics. It appears off-white or cream-coloured, opaque, and lacks lustre. ABS is known for its toughness and hardness, making it challenging to break. Polymethyl Methacrylate (PMMA), in contrast, is transparent like glass, can be bent when heated, and is workable by hand.
In addition to visual and tactile characteristics, plastics can be differentiated by their behaviour in liquids. When dropped into a liquid, a plastic will float or sink depending on the relative densities of the plastic and the liquid. This behaviour can be observed to distinguish between different types of plastics.
Lastly, the recycling symbols on plastic products can provide valuable information about the type of plastic. These symbols, usually found at the bottom of containers, indicate the specific type of plastic used, allowing for easier identification.
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Burning test
While the burn test is a useful method for identifying plastics by their density, it should be used with caution due to the toxicity of many plastic combustion products. If you are confronted with an unknown plastic, it is recommended to first look for a label. Since 1988, the SPI resin identification coding system has been widely adopted by plastics manufacturers to facilitate post-consumer recycling. If your sample does not have a label, the burn test can be used to correlate a plastic sample's composition with a set of observable properties.
To perform the burn test, hold a sample of the plastic to the edge of a flame until it ignites. If no flame is produced immediately, hold the sample in the flame for about 10 seconds. Observe the nature of the smoke and the presence of soot in the air. Note whether the sample drips while burning. Extinguish the flame and cautiously smell the fumes, comparing the odour to that of a known sample if possible.
The observable properties of the burn test can be used to identify the plastic's composition. For example, ABS tubing is characterised by a free-burning flame colour and smoke. Polyethylenes and polypropylenes float in water (density = 1 g/mL), while most other plastics sink. Among heavier-than-water plastics, ABS and PS will float in glycerin (density = 1.26 g/mL), but PETE and PVC will not.
It is important to note that the burn test should only be used for tentative identification, as some complex plastic compounds require a rigorous analysis for definitive identification. After performing the burn test and making a tentative identification, it is recommended to conduct one additional test to confirm the results.
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Frequently asked questions
The density of a plastic can be determined by weighing a cubic centimetre of it and calculating the mass per unit volume. This can be done by measuring mass and volume and dividing volume into mass.
The six commonly used types of plastics can be identified by their recycling symbols, which are usually found at the bottom of the container. In China, the symbol is a two-digit number with a leading "0", while foreign symbols are a single digit without the "0".
Yes, to an extent. For example, Polypropylene (PP) is a white, semi-transparent, waxy substance that feels quite smooth to the touch. Polyethylene (PE) is a white powder or transparent particulate resin that has a wax-like smooth touch.
Yes, different plastics exhibit distinct combustion characteristics. For example, if the smell of the burning plastic is that of burnt wool, the flame is blue with a yellow tip, the molten plastic drips, and it does not continue to burn when the flame source is removed, then it is likely to be nylon.
Yes, the density of different types of plastics varies, leading to different sinking and floating behaviours in water and other solutions. For example, heavy plastics sink in a brine (salt) solution, while lighter ones float.
