Maya Ellison
The density of an object determines whether it will sink or float when placed in water. If an object is denser than water, it will sink. Plastics are made from polymers, and there are many different types of plastics with varying densities. Some plastics are denser than water and will sink, while others are less dense and will float. For example, HDPE, LDPE, and PP plastics are less dense than water and therefore float, whereas PET, PVC, and PS plastics are denser and will sink. The density of a plastic can also vary depending on its thickness and shape.
| Characteristics | Values |
|---|---|
| Plastics that are heavier than water | Poly(vinyl chloride) (PVC), Poly(ethylene terephthalate) (PET), Poly(methyl methacrylate) (PMMA), Polyvinyl chloride (PVC), Polystyrene (PS), Polyethylene terephthalate (PETG), Polycarbonate, Nylon |
| Density of water | 1 gram per cubic centimeter |
| Density of heavier plastics | Greater than 1 gram per cubic centimeter |
| Density values of some heavier plastics | PVC = 1.388, PET = 1.333, PMMA = 1.159, PETG = 1.27 grams per cubic centimeter, Nylon = 1.14-1.15 grams per cubic centimeter |
| Other characteristics | Some heavier plastics have a matte finish and are easy to paint or laminate, like PVC. Nylon is used in clothing and carpets and has a characteristic smell when burnt. |
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What You'll Learn
Plastic density determined by volume and mass
The density of a plastic sample may change according to several factors. Density is defined as the total mass of an object divided by its total volume, usually expressed in grams per cubic centimetre (g/cm3). The density of water is 1 gram per cubic centimetre.
The density of plastics depends on the crystalline structure of the polymers. Amorphous polymers have a random molecular structure, resulting in lower density. Crystalline polymers, on the other hand, have a systematic and compact arrangement of polymer chains and crystals, leading to a higher density. Additionally, density is inversely proportional to temperature. As the temperature increases, materials tend to expand, increasing their volume and decreasing their density. Other factors, such as the loss of plasticizers and absorption of solvents, can also influence the density of plastics.
To determine the exact density of a plastic, various methods can be employed. One common method is the ASTM D1505, which uses a density-gradient technique. This 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 then calculated based on its relative position to the floats and the densities of the glass beads. Another method, the ASTM D792-98, determines the specific gravity and density of solid plastics in forms such as sheets, rods, tubes, or moulded items.
Different types of plastics have different densities. For example, high-density plastics such as PET (polyethylene terephthalate) and PVC (polyvinyl chloride) have densities greater than water and will sink. In contrast, plastics with lower densities, such as HDPE (high-density polyethylene) and LDPE (low-density polyethylene), will float.
The density of plastics is an important parameter that affects their applications. High-density polymers are used in applications requiring structural strength, chemical resistance, and higher temperatures, such as pipes, automotive parts, and industrial containers. On the other hand, low-density polymers provide better flexibility, impact resistance, and processing ease.
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Floating and sinking plastics
Whether a plastic object floats or sinks in water depends on its density relative to water. If the density of the plastic is lower than that of water, it will float; if it is denser, it will sink. The density of water is one gram per cubic centimetre.
Some plastics, such as HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene), have densities lower than that of water and therefore float. These plastics are often used for containers and plastic bags. Other plastics, such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene), have higher densities and will sink in water. PET, for example, is commonly used for single-use plastic drink bottles, and its density is 1.38-1.39. This means that many un-recycled bottles end up sinking to the ocean floor.
The density of plastics can vary depending on their form and thickness. For example, the density of nylon is typically given as 1.15, but this may refer to solid pellets or bricks of nylon, whereas the floating bits of plastic in the ocean may have a different density due to their shape. Nylon used in clothing and carpets has a density of 1.14 and will sink. Similarly, the density of PVC depends on its thickness, with sheets of varying thickness measuring between 0.55 and 0.70 grams per cubic centimetre.
The presence of salt in water also affects the density of the water and, consequently, the ability of plastics to float or sink. Salt water is denser than pure water, so plastics will float better in the ocean than in freshwater.
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Plastic polymers with high relative density
The density of a plastic sample may change according to several factors, including the crystalline structure of the polymers. Crystalline polymers have a compact arrangement of polymer chains and crystals, resulting in a higher crystalline structure and, consequently, higher density. Density is inversely proportional to temperature; as the temperature increases, the material's volume increases, reducing its density. Other factors that can alter the density of plastics are the loss of plasticizers and the absorption of solvents.
Some plastic polymers have a relative density greater than one, meaning they are denser than water. Poly(vinyl chloride) or PVC, for instance, has a relative density of 1.388, while Poly(ethylene terephthalate) or PET measures 1.333. Poly(methyl methacrylate) or PMMA has a relative density of 1.159. These plastics will sink in water.
Polyvinyl chloride (PVC) is a popular choice for signs, exhibits, and displays due to its smooth matte finish, which facilitates painting and lamination. It also offers excellent screw and staple retention. PVC is one of the lightest plastics available, although its density depends on its thickness.
High-density polyethylene (HDPE) is another plastic polymer with a high strength-to-density ratio. Its density ranges from 930 to 970 kg/m3, and it is commonly used in the production of plastic bottles, corrosion-resistant piping, geomembranes, and plastic lumber. HDPE is also used in cell liners in sanitary landfills to prevent soil and groundwater pollution. Additionally, the pyrotechnics trade favours HDPE mortars over steel or PVC tubes due to their durability and safety.
Vivak, a specific form of polyethylene terephthalate (PETG) created by Bayer, has a density of 1.27 grams per cubic centimeter. It is known for its impact and temperature resistance, as well as its ability to be die-cut and punched without shattering, making it ideal for displays and complex shapes.
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Plastic weight and specific gravity
The weight of plastics is an important consideration in material science and engineering. Weight determines whether a plastic is suitable for a particular application, and whether other components can support the plastic.
Specific gravity is a measure of density, or how heavy an object is compared to water. Water has a specific gravity of 1, meaning it weighs one gram per cubic centimetre. A material with a specific gravity of 1.5, for example, would weigh 1.5 grams per cubic centimetre.
Plastics generally have specific gravities ranging from 0.9 to 2.2, which is less than water's specific gravity. However, some plastics, like PVC or PET, may have a higher specific gravity due to added fillers or additives that increase density. For example, Poly(vinyl chloride) [PVC] has a specific gravity of 1.388, and Poly(ethylene terephthalate) [PET] is 1.333.
Some other specific gravity values for plastics include Vivak (a form of PETG) at 1.27 grams per cubic centimetre, and ABS sheet at 1.03 grams per cubic centimetre. Polyvinyl chloride (PVC) is one of the lightest plastics available, with sheets from one to six millimetres measuring 0.70 grams per cubic centimetre, and those ranging from 10 to 25 millimetres weighing 0.55 grams per cubic centimetre.
Specific gravity is important in plastic recycling, as it allows for the separation and identification of various types of plastics during the recycling process. It is also useful in manufacturing, as it can be used to estimate quality and further application in a particular field.
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Plastic weight and recycling codes
The weight of plastic is measured in grams per cubic centimetre. Different types of plastics have different densities and weights, and these can be identified using resin or recycling codes.
Plastic Recycling Codes
Plastic recycling codes, also known as resin codes or plastic resin identification codes, are used to identify the type of plastic a product is made of and provide instructions for recycling. The seven most common plastic recycling codes are numbered from 1 through 7 and are found inside the Mobius symbol, which is a triangle made of arrows.
- Code 1: PET or PETE (polyethylene terephthalate) is a common plastic for single-use bottled beverages due to its lightweight, inexpensive, and easy-to-recycle nature. It has a density higher than water and a specific gravity of 1.333.
- Code 2: HDPE (high-density polyethylene) is used in plastic shopping bags and can often be returned to stores for recycling. It has a lower density than water.
- Code 4: LDPE (low-density polyethylene) is not commonly recycled through curbside programs, but some communities may accept it. LDPE products include plastic bags and toothpaste tubes. It has a lower density than water.
- Other codes: Other plastic recycling codes include PVC (polyvinyl chloride), which is known for its lightweight properties, and PMMA (Poly(methyl methacrylate)), which has a density of 1.159.
It is important to note that the specific gravity or density of a plastic does not always determine whether it will float or sink. The shape of the object and the type of water (saltwater or freshwater) can also influence buoyancy.
By understanding these plastic recycling codes, you can ensure that you place items in the proper bins for recycling. However, it is always recommended to check your local municipality's recycling guidelines for specific instructions on what is accepted and how to prepare items for recycling.
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Frequently asked questions
Some plastics that are heavier than water include Poly(vinyl chloride) (PVC) with a density of 1.388, Poly(ethylene terephthalate) (PET) with a density of 1.333, and nylon with a density of 1.15 and 1.14.
The density of an object can be compared to that of water to determine whether it will sink or float. If the density of the plastic is greater than 1 gram per cubic centimeter, it is denser than water and will sink.
High-density plastics such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene) sink in water due to their higher densities.
Yes, plastics with densities less than that of water will float. Examples include HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene).
