Maya Ellison
Whether a piece of plastic floats or sinks depends on its density relative to water. While most plastics are denser than water, some plastics with a lower density will float. For example, HDPE, LDPE, and PP plastics will float, while PET, PVC, and PS will sink. Salt water is denser than pure water, so plastics float better in the ocean. However, microplastics, or plastic debris smaller than 5mm in diameter, are a growing concern due to their presence in marine environments and potential impact on ecosystems.
| Characteristics | Values |
|---|---|
| Plastics that float in hot water | HDPE, LDPE, PP |
| Plastics that sink in hot water | PET, PVC, PS, nylon |
| Reason for floating | Density of plastic is less than that of water |
| Reason for sinking | Density of plastic is more than that of water |
| Density of HDPE, LDPE, PP | Less than that of water |
| Density of PET, PVC, PS, nylon | More than that of water |
| Density of saltwater | More than that of pure water |
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What You'll Learn
HDPE, LDPE, and PP float
Whether or not an object floats in water depends on its density relative to the water. If an object is less dense than water, it will float, whereas if it is denser, it will sink.
HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene) are types of plastics that are less dense than water and, therefore, float. These plastics are commonly used to make containers and plastic bags.
HDPE pipes, for example, are used in water applications and have been successfully deployed for 50 years. They are slightly less dense than water and will float even when filled with water. This buoyancy makes them useful for applications where pipes need to work above water or slurry.
LDPE is another type of plastic that is often used in the manufacture of plastic bags and containers. Its lower density compared to water allows it to float, making it suitable for specific applications where buoyancy is required.
PP, or polypropylene, is also a commonly used plastic that exhibits buoyancy due to its lower density relative to water. This property enables its use in various applications, including the creation of floating devices and water-based packaging solutions.
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Density is key
Plastics are made up of polymers and most types are denser than water. However, some plastics have a density lower than that of water and will therefore float. These include HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene). These plastics are often used for containers and plastic bags.
On the other hand, plastics with a higher density than water, such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene), will sink. PET, for example, is commonly used for single-use plastic drink bottles, while PVC is used for sheeting, rope, and medical tubing.
It is worth noting that the density of plastics can vary depending on their shape and how they are manufactured, so the floating behaviour of a plastic object may not always be immediately obvious. For instance, the floating bits of plastic you see in the ocean might have a lower density due to their shape, causing them to float despite the material typically sinking.
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Salt water is denser than pure water
The density of a plastic item determines whether it will float or sink in water. If an object displaces less water than its own mass when fully submerged, it is denser than the water and will sink. If it displaces the same amount of water or more, it will float.
Most plastics are denser than water, but some types of plastic are less dense and will float. For example, HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene), which are often used in containers and plastic bags, have a lower density than water and will float. On the other hand, higher-density plastics like PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene solid) will sink.
The density of water itself can vary. Salt water is denser than pure water. This is why people float higher in saltwater than in freshwater. When a person is in saltwater, they displace less water volume than they would in freshwater, and the buoyant force is greater. As a result, a smaller portion of their body is submerged, and they float more easily.
The difference in density between saltwater and freshwater has interesting implications for objects floating in the ocean. Since saltwater is denser, things float better in the oceans. This is important to consider when thinking about plastics in the ocean, as some plastics will float in saltwater even if they would have sunk in freshwater.
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Higher-density plastics sink
While plastics are denser than water, not all plastics sink. The density of an object determines whether it will sink or float in water. If an object's density is greater than that of the water, it will sink, whereas if its density is less than that of the water, it will float.
Some plastics, such as HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene), have a lower density than water and, therefore, float. These plastics are commonly used for containers and plastic bags.
However, higher-density plastics, such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene), have a higher density than water and, thus, sink. PET, with a density range of 1.38-1.39, is often used for single-use plastic drink bottles. Many of these bottles end up in the oceans, sinking to the bottom. PVC, the second most widely used plastic globally, is used for sheeting, rope, blood bags, medical tubing, siding, and flooring, among other applications. PS, with a density of 1.05 to 1.07, is commonly used for hot cups and clam shells for hamburgers and other takeaway foods.
It is worth noting that the density of plastics can vary during manufacturing, resulting in a range of density values for each type of plastic. Additionally, the shape of plastic objects can affect their buoyancy, as different shapes can displace varying amounts of water.
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Nylon will sink
Density is calculated using the formula M/V. When an object is submerged in water, if it displaces less water than its own mass, it is denser than water and will sink. If the mass of the object is less than the volume of water it displaces, it will float.
Nylon has a density of 1.15, whereas water has a density of 1. This means that nylon displaces less water than its own mass when submerged and will therefore sink.
It is worth noting that the shape of an object can also affect whether it floats or sinks. For example, some plastics may float due to their shape, despite being denser than water. Additionally, salt water is denser than pure water, so objects float better in the ocean.
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Frequently asked questions
Plastics that are less dense than water float.
HDPE, LDPE, and PP float in water.
Higher-density plastics such as PET, PVC, and PS do not float in water.
