Alexei Volkov
Plastics are a major source of pollution in the marine environment, with microplastics—small plastic debris less than 5mm in diameter—being of particular concern due to their ubiquitous presence and limited knowledge of their ecological effects. The buoyancy of plastics depends on their density relative to water and the shape of the plastic item. Higher-density plastics such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene solid) sink, while lower-density plastics like HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene) float.
| Characteristics | Values |
|---|---|
| Density | Higher-density plastics tend to sink, while lower-density plastics tend to float |
| Specific Plastics | HDPE, LDPE, and PP float; PET, PVC, and PS sink |
| Shape | The shape of an object can affect whether it sinks or floats, independent of density |
| Water Type | Objects float better in salt water than in pure water |
| Biofouling | Plastics can become denser after time at sea due to biofouling |
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What You'll Learn
Density determines floatation
Density is defined as the mass per unit volume of a substance. It is denoted by the Greek letter rho (ρ). The density of plastics varies depending on their composition and structure. For instance, HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene) have lower densities than water, causing them to float. Conversely, plastics with higher densities, such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene solid), sink in water.
The buoyancy of an object in a fluid, such as water, is determined by its density relative to the fluid. An object will float if it displaces an equal or greater volume of fluid than its own volume. In other words, if the mass of the object is less than the mass of the fluid it displaces, it will float. Conversely, if an object displaces less volume of fluid than its own volume when fully submerged, it is denser than the fluid and will sink.
The shape of an object can also influence its buoyancy. An object may float even if it is denser than the fluid due to its shape, as it may not be heavy enough to break the surface tension of the fluid. Additionally, salt water is denser than pure water, allowing objects to float more easily in the ocean.
The density of plastics can be determined through various methods, such as ASTM D1505 and ASTM D792-98. ASTM D1505 involves placing the material in a liquid column of variable density with standard floats, usually glass beads of known density. The density of the material is calculated based on its relative position to the floats. ASTM D792-98 is used to determine the specific gravity and density of solid plastics in forms such as sheets, rods, tubes, or molded items.
The density of plastics is an important factor in their applications. High-density polymers offer greater strength, temperature resistance, and chemical resistance, making them suitable for pipes, automotive parts, and industrial containers. On the other hand, low-density polymers provide better flexibility, impact resistance, and ease of processing, making them a cost-effective choice for applications requiring flexibility and simple manufacturing processes.
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HDPE, LDPE, PP float
Although plastics are denser than water, their shape can affect whether they sink or float. For example, plastic items with a lower mass than the volume of water they displace will float.
HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene) are types of plastics that make up containers and plastic bags. These plastics have a density lower than that of water and, therefore, float.
HDPE, LDPE, and PP are commonly used in industrial applications due to their buoyancy and durability. For instance, HDPE is used in the construction of floating structures such as docks, boat floats, and industrial pump floats. LDPE, being a lighter variant of HDPE, is often used in the manufacturing of plastic bags, containers, and flexible tubing.
On the other hand, higher-density plastics like PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene solid) tend to sink in water due to their higher density.
It is important to note that the presence of these plastics in our oceans and waterways can have detrimental effects on marine life and the environment. While these plastics may float and not immediately sink, they can still cause harm by releasing toxic chemicals or being ingested by marine creatures.
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Higher-density plastics sink
Whether or not a plastic object floats depends on its density relative to water. If an object is less dense than water, it will float; if it is more dense, it will sink.
However, the shape of the object also matters. An object that is denser than water may still float if it does not break the water's surface tension.
Some plastics, such as HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene), are less dense than water and therefore float. Other plastics, such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene solid), are more dense and will sink.
It is worth noting that plastics in the ocean can become more dense over time due to biofouling, and may eventually sink as a result.
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Salt water density
The density of seawater is influenced by its temperature, salinity, and pressure. The density increases as salinity increases and temperature decreases. Seawater is denser than both freshwater and pure water due to the dissolved salts increasing the mass by a larger proportion than the volume. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one litre by volume) of seawater has approximately 35 grams (1.2 oz) of dissolved salts, predominantly sodium (Na+) and chloride (Cl-) ions. The density of surface seawater ranges from about 1020 to 1029 kg/m3, depending on temperature and salinity.
The freezing point of seawater decreases as salt concentration increases. At typical salinity, it freezes at about −2 °C (28 °F). The addition of salt to water lowers the freezing temperature, and this is why sea water does not start forming the lattice structure that lowers its density until it is much colder than fresh water. The density of a material is given in units of mass per unit volume and expressed in kilograms per cubic metre in the SI system of units. In oceanography, the density of seawater has been expressed historically in grams per cubic centimetre.
The saltiest locations in the ocean are regions where evaporation is highest or in large bodies of water with no outlet to the ocean. The Red Sea and the Persian Gulf region are the saltiest ocean waters, with very high evaporation and little freshwater inflow. Seawater salinity is expressed as a ratio of salt (in grams) to the litre of water, written in parts per thousand (ppt). The salinity in isolated bodies of water can be considerably greater—about ten times higher in the case of the Dead Sea.
Ocean salinity has been stable for billions of years, likely due to a chemical/tectonic system that removes as much salt as is deposited. Climate change, rising levels of carbon dioxide, excess nutrients, and pollution are altering oceanic geochemistry. The use of seawater for agricultural purposes has been considered in areas with higher regions of sand dunes, such as Israel, as it would eliminate substantial costs associated with freshwater. One of the most successful plants in saltwater agriculture is the halophyte, a salt-tolerant plant whose cells are resistant to the detrimental effects of salt in the soil.
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Biofouling increases density
The buoyancy of plastics in water depends on their density and shape. Some plastics, such as HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene), have a lower density than water and, therefore, float. On the other hand, higher-density plastics like PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene solid) sink in water.
However, the presence of biofouling can alter the buoyancy of plastics, even those with a lower density than water. Biofouling is the accumulation of microorganisms, plants, algae, or small animals on surfaces where they are not wanted, such as on plastics in water. This accumulation increases the density of the plastic, causing it to sink.
Research has shown that biofouling communities, including bacteria and eukaryotes, can colonize plastic over time, increasing their density and causing them to sink. This was observed in the coastal waters of the North Sea, where biofouling caused polyethylene microplastics to sink, even during winter when growth rates are lower.
Biofouling has significant economic impacts, particularly in the shipping industry. When biofouling occurs on a ship's hull, it increases the hydrodynamic volume, frictional effects, and drag, leading to reduced vessel performance and increased fuel consumption. Additionally, in aquaculture, biofouling increases production and management costs while decreasing product value. It can compete with shellfish for food and oxygen, impede water flow, and interfere with valve operations, negatively affecting stock growth, condition, and survival.
To mitigate the effects of biofouling, industries, and governments spend billions of dollars annually on prevention and control measures. These include the development of antifouling coatings and treatments, such as biocide paints and non-toxic hydrophilic coatings, which prevent the attachment of microorganisms and biofilm formation.
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Frequently asked questions
Higher-density plastics such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene solid) sink in water.
An object that floats will displace the same volume of water as its mass. If an object displaces less volume of water than its mass, it is denser than the water and will sink.
No, some plastics have a lower density than freshwater and will float. However, salt water is denser than freshwater, so plastics are more likely to float in the ocean.
