Alexei Volkov
The density of plastic in relation to water is an important topic, especially when considering plastic waste in the ocean. Some plastics are denser than water and will sink, while others are less dense and float. The density of seawater is around 1.027, and generally, anything with a lower density will float, and anything higher will sink. Polyethylenes and polypropylenes have a density of about 0.900 to 0.970, so they float. In contrast, other plastics like styrenes, nylons, polyesters, and vinyls have higher densities, ranging from 1.050 to 1.440, causing them to sink. It's worth noting that some plastics become denser over time due to biofouling or collecting sediment, which can cause them to sink even if they initially floated.
| Characteristics | Values |
|---|---|
| Density of water | 1 gram per cubic centimeter |
| Density of plastics | Varies, most plastics are denser than water |
| Examples of plastics with density lower than water | HDPE, LDPE, PP |
| Examples of plastics with density higher than water | PET, PVC, PS, styrenes, nylons, polyesters, polyurethanes, vinyls |
| Plastic density changes over time | Plastics tend to become denser after time at sea due to biofouling and collecting sediments |
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What You'll Learn
HDPE, LDPE, and PP float
The density of plastics varies, with some plastics denser than water and others less dense. For an object to float on water, it must displace an equal or larger mass of water than its own mass. If an object is denser than water, it will sink unless it displaces enough water to balance its weight.
HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene) are examples of plastics with densities less than that of water, allowing them to float. These plastics are commonly used in containers, plastic bags, and food packaging. Polypropylene, for instance, is used in chocolate and chip wrappers, beverage bottles, and dishwasher-proof food containers.
The buoyancy of these plastics can be attributed to their low density relative to water. When submerged, they displace a volume of water equal to or greater than their own mass, resulting in them floating on the surface.
It is important to note that the shape of an object can also influence its buoyancy. Even if an object has a higher density than water, it may still float due to its shape, as it may not be able to sink below the water's surface to displace enough water to balance its weight.
Additionally, the presence of salt in water can impact buoyancy. Saltwater is denser than pure water, making it easier for objects to float in the ocean or other saltwater bodies.
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PET, PVC, and PS sink
When it comes to plastics and their interaction with water, density plays a crucial role in determining whether they will sink or float. Most plastics are denser than water, yet surprisingly, they don't always sink. So, what's the reason behind this seemingly contradictory behaviour?
Let's focus on three specific types of plastics: PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene solid). These plastics are considered high-density plastics, and unlike their lower-density counterparts like HDPE, LDPE, and PP, they do sink in water.
To understand why PET, PVC, and PS sink, we need to grasp the fundamental concept of buoyancy. For any object to float on water, it must displace an equal or greater mass of water compared to its own mass. In simpler terms, if an object weighs more than the amount of water it pushes aside, it will sink. This principle applies to both solids and liquids.
Now, let's apply this knowledge to our three plastics. PET, PVC, and PS have densities higher than that of water. When placed in water, they will displace a volume of water equal to their own volume. However, since their masses are greater than the mass of the displaced water, they sink. It's as simple as that!
It's worth noting that the shape of an object can also influence its buoyancy. Even if an object is denser than water, its shape might prevent it from sinking immediately. Additionally, the presence of surface tension in water can also hinder an object's ability to break through and sink. These factors, along with the density differences, help explain why some plastics seem to defy density expectations and float instead of sink.
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Nylon has a density of 1.14-1.15
Nylon is a synthetic polymer with a density typically ranging from 1.14 to 1.15 g/cm³. This density is higher than that of water, which has a density of 1 g/cm³. Due to its higher density relative to water, nylon sinks when submerged.
The density of nylon can vary slightly depending on the specific type of nylon and the manufacturing process used. Different types of nylon may have slightly different densities due to variations in their chemical composition and structure.
The density of nylon is influenced by several factors, including crystallinity and the degree of polymerization. Crystallinity refers to the orderly arrangement of molecules within the material. Nylon with a higher level of crystallinity tends to be denser because the molecules are packed more closely together. The polymerization process used to produce nylon, such as ring-opening polymerization or condensation polymerization, also affects crystallinity and, consequently, the density of the material.
The degree of polymerization in nylon refers to the number of repeating units or monomers in a polymer chain. A higher degree of polymerization results in longer polymer chains and a denser material. Factors such as reaction conditions during polymerization, the molecular weight and structure of the monomers, and the use of chain-extending agents all influence the degree of polymerization and, thus, the density of nylon.
The density of nylon is an important factor in determining its performance in various applications. For example, in fishing nets, a dense material like nylon is desirable as it sinks and aids in catching fish. However, in buoyancy devices, a lower-density material is preferred to maintain flotation.
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Lighter plastics become denser in the ocean
Plastic pollution in the ocean is a pressing issue. While plastics with a higher density than water, such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene solid), sink, it is surprising that plastics with a lower density than water often end up sinking in the ocean as well.
The density of seawater is around 1.027, and generally, objects with a lower specific gravity will float, while those with a higher value will sink. Polyethylenes and polypropylenes, which are commonly used for containers and bags, have a specific gravity of about 0.900 to 0.970, causing them to float. However, research has revealed that these lighter plastics, which initially float, eventually become denser after spending time in the ocean and end up sinking.
This transformation can be attributed to a process known as biofouling, where the plastics collect sediment and other substances over time, increasing their density. This phenomenon is particularly notable in the case of polyethylene grocery bags, which are rarely found floating in the deep ocean. Instead, they tend to sink due to the accumulation of sediment.
The impact of plastic pollution extends beyond the ocean's surface, with only about 1% of plastic litter remaining at the surface. The remaining 99% has sunk beneath the surface or settled on the seafloor. This submerged plastic debris is influenced by deep-sea circulation, leading to the creation of seafloor hotspots with extremely high concentrations of plastic pieces.
Furthermore, the shape of plastic items plays a role in their buoyancy. Even if a plastic object has a higher density than water, its shape may prevent it from sinking. Additionally, the surface tension of water can also influence whether an object floats or sinks, as it must be heavy enough to break the surface tension to submerge.
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Polyethylenes and polypropylenes have a specific gravity of 0.900-0.970
Polyethylenes and polypropylenes are versatile polymers with numerous applications. Polyethylenes, for instance, are used in making heavy-duty sacks, refuse bags, carrier bags, general packaging, and in the building and construction industry. They are also used as gelling agents in mineral oils for lubricating greases.
The specific gravity of polyethylenes and polypropylenes typically falls within the range of 0.900-0.970. This value can vary depending on the type of polyethylene or polypropylene in question. For instance, low-density polyethylene (LDPE) has a specific gravity ranging from 0.915 to 0.94, while high-density polyethylene (HDPE) can have a specific gravity as high as 0.97. Polypropylene films, on the other hand, typically have a specific gravity between 0.90 and 0.91.
The specific gravity of a material is an important factor in determining its quality and applications in various fields. For example, in the context of plastics, specific gravity is used in the recycling process to sort mixed plastic polymer types. It is also crucial in designing and engineering plastic products, as it helps predict their behaviour in different working environments.
It is worth noting that while most plastics have a specific gravity lower than that of water (ranging from 0.9 to 2.2), some plastics, such as PVC and PET, may have a higher specific gravity due to the addition of fillers or additives. These higher-density plastics will sink in water, while plastics with a lower specific gravity, like LDPE, LLDPE, and PP, will float.
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Frequently asked questions
Plastics are synthetic materials made from organic polymers. They are derived from natural materials, such as coal, natural gas or crude oil.
The density of water is 1. If the plastic has a density greater than 1, it will sink. If it has a density less than 1, it will float.
The plastics with a density greater than 1 include PET (polyethylene terephthalate), PVC (polyvinyl chloride), PS (polystyrene), nylon, styrene, polyester, polyurethane, and vinyl.
