Maya Ellison
Plastic is a versatile material used in a wide range of products, from clothing to containers. While it may seem intuitive that plastic, being artificial, would be denser than water, this is not always the case. In fact, most plastics are less dense than water and will float. This is due to the relative densities of the materials that comprise them. For example, nylon, a common plastic with a density of around 1.14, will sink in water, while low-density polyethylene, used in trash bags, will float due to its lower density. Understanding the density of plastics is crucial for applications ranging from recycling to marine pollution mitigation.
| Characteristics | Values |
|---|---|
| Plastics with a lower density than water | HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene) |
| Plastics with a higher density than water | PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene solid) |
| Nylon density | 1.14-1.15 |
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What You'll Learn
LDPE, HDPE, and PP plastics float
There are many different types of plastics, each with varying densities. LDPE (low-density polyethylene), HDPE (high-density polyethylene), and PP (polypropylene) are three types of plastic that float on water due to their lower density compared to water. These plastics are commonly used in the production of containers and plastic bags.
LDPE is a type of plastic with a lower density than HDPE and is often used for trash bags and plastic containers that previously held laundry soap or other coloured containers. It has a density lower than that of water, allowing it to float. The density of LDPE can vary during manufacturing, resulting in a range of values.
HDPE, or high-density polyethylene, is used in various products, including milk jugs, plastic containers, and laundry soap bottles. Despite its name, HDPE has a lower density than water, enabling it to float. Like LDPE, the density of HDPE can also vary during manufacturing, influencing its floating capabilities.
PP, or polypropylene, is another plastic with a density lower than that of water, allowing it to float. PP is known for its versatility and is used in a wide range of applications, from packaging to textiles and automotive components.
It is important to note that the buoyancy of an object in water depends on its ability to displace an equal or larger mass of water compared to its own mass. This principle applies to plastics as well. Additionally, the shape of an object can also affect its buoyancy, as different shapes can displace water differently, even if the material's density remains constant.
In contrast to LDPE, HDPE, and PP, there are higher-density plastics such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene) that sink in water due to their higher density. These plastics are commonly used in products like single-use drink bottles, sheeting, and medical tubing.
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Higher-density plastics sink
While it is commonly understood that wood, paper, and most plastics are less dense than water, and will therefore float, there are some higher-density plastics that sink.
The density of a material is defined as its mass per unit volume, which tells us how much matter is packed into a given space. Objects with a density greater than water (1 gram per cubic centimeter) will sink, while those with a lower density will float.
Some plastics, such as HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene), which are commonly used for containers and plastic bags, have a density less than that of water and will float.
However, there are higher-density plastics that will sink. These include PET (polyethylene terephthalate), with a density of 1.333, PVC (polyvinyl chloride), with a density of 1.388, and PS (polystyrene solid). These plastics are denser than water and will, therefore, sink.
The density of a plastic can vary depending on its shape and form. For example, nylon has a density of 1.14 and will sink, but floating bits of nylon, such as plastic bags, might have a different density due to their shape.
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Nylon has a density of 1.14-1.15
Nylon, the first commercially successful synthetic thermoplastic polymer, has a density of 1.14-1.15 g/cm3. This density range is typical for unreinforced nylon, with the lower figure representing the density of unreinforced nylon and the higher figure representing the density of nylon reinforced with 30% glass.
Nylon was invented by researchers at DuPont, with the first nylon, nylon 66, being synthesised on February 28, 1935, by Wallace Hume Carothers. It was produced using a cold drawing method by Carothers' coworker, Julian W. Hill, and exhibited the desired properties of elasticity and strength. However, its complex manufacturing process and high density compared to other plastics may be considered drawbacks.
As a thermoplastic, nylon can be melt-processed into fibres, films, and diverse shapes. It is commonly used in textiles, fishing lines, carpets, and food packaging due to its toughness and low gas permeability. Nylon is also used in engineering applications, such as machine screws, gears, and gaskets, where its strength and heat resistance are advantageous.
The density of nylon is an important factor in determining its buoyancy. In general, for an object to float on water, it must displace an equal or larger mass of water than its own mass. Since nylon's density is greater than that of water, it will typically sink in water unless it has a shape that allows it to displace a sufficient volume of water to remain afloat.
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Density is mass per unit volume
Density, denoted by the Greek letter rho (ρ) or the Latin letter D (or d), is defined as the ratio of a substance's mass to its volume. Mathematically, this is expressed as:
> ρ = M/V
Where ρ is density, M is mass, and V is volume. The density of a substance is commonly expressed in units of grams per cubic centimetre (g/cm^3). For example, the density of water is 1 gram per cubic centimetre, while Earth's density is 5.51 grams per cubic centimetre.
The weight of a body, which is usually of more practical interest than its mass, can be obtained by multiplying the mass by the acceleration of gravity. Density provides a convenient means of determining the mass of a body from its volume or vice versa. For instance, the mass of an object can be calculated by multiplying its volume by its density (M = V*d), and the volume can be determined by dividing the mass by the density (V = M/d).
The density of a substance can vary across different regions of the same object if the object is not homogeneous. In such cases, the density at a specific location can be calculated by considering the density of a small volume around that location. For bulk materials, such as sugar, sand, or snow, the presence of voids, which are regions containing something other than the material of interest, can affect the density. The voids can be filled with air, vacuum, liquid, solid, or a different gas or gaseous mixture. To determine the volumetric mass density, the volume of the void fraction must be discounted, which can be done through geometrical reasoning or empirical methods.
The density of a fluid can be altered by mixing it with another substance. For example, adding salt to water increases its density, causing eggs to float on the surface. This principle is also believed to be the reason why objects float on the Dead Sea, a lake in Palestine with a high salt content.
Now, let's consider the density of plastics in relation to water. Most plastics are denser than water, with densities ranging from 1.15 (nylon) to 1.388 (PVC). However, certain types of plastics with lower densities, such as LDPE (low-density polyethylene) and PP (polypropylene), are less dense than water and will float. On the other hand, higher-density plastics like PET (polyethylene terephthalate) and PVC (polyvinyl chloride) will sink.
In general, for an object to float on water, it must displace an equal or larger mass of water than its own mass. If an object displaces less water than its own mass when fully submerged, it is denser than water and will sink.
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Salt water is denser than pure water
It is true that salt water is denser than pure water. This is because the dissolved salts in salt water increase the mass of the water by a larger proportion than the volume. The density of seawater is 1.025 kg/L, while the density of pure water is 1.0 kg/L at 4 °C (39 °F).
The salinity of seawater, or salt water, is approximately 3.5% (35 g/L), meaning that every kilogram (roughly one liter by volume) contains about 35 grams of dissolved salts, predominantly sodium (Na+) and chloride (Cl-) ions. These salts are carried into the sea by rivers after rainfall washes them out of the ground. Upon reaching the ocean, the salts become concentrated as more salt arrives over time. This process, known as "continental weathering," was first proposed by Sir Edmond Halley in 1715. In addition to the salts carried by rivers, sodium is also leached out of the ocean floor, and chloride is released from the Earth's interior as hydrochloric acid.
The higher density of salt water has several interesting effects. For example, it causes things to float better in the oceans. It also results in a lower freezing point, with typical seawater freezing at about −2 °C (28 °F). The freezing point continues to decrease as salt concentration increases. Additionally, large-scale desalination of seawater is often used to provide water for agriculture in dry, desert environments. However, drinking seawater is counterproductive for maintaining hydration, as the body excretes more water to eliminate the salt than the amount of water obtained from the seawater.
While most plastics are denser than pure water, there are some types of plastics with lower densities that do float, such as HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene), which are commonly used for containers and plastic bags.
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Frequently asked questions
LDPE (low-density polyethylene), HDPE (high-density polyethylene), and PP (polypropylene) are plastics that are less dense than water and will float.
To determine if an object will float or sink in water, you need to understand the concept of density. Density is defined as mass per unit volume. Objects with a density greater than water (1 gram per cubic centimeter) will sink, while those with a lower density will float.
Wood and paper are also less dense than water and will float.
