The term “plastic” describes a large category of synthetic materials, not a single substance, meaning there is no single answer to the question of its density. Density is a physical property that measures how much mass is packed into a given volume, and it is a defining characteristic for every specific type of plastic. This property is expressed as mass per unit volume, typically in grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³). Understanding this value reveals a great deal about a material’s potential uses, from the structural strength of a car part to the floatability of a water bottle.
Understanding Mass and Volume
Density combines two fundamental properties of matter: mass and volume. Mass refers to the total amount of matter an object contains, while volume is the three-dimensional space that object occupies. Density is calculated by dividing mass by volume. For example, if a small rock and a large feather pillow have the same mass, the rock must have a much smaller volume, resulting in high density. In the world of polymers, density reveals how efficiently the microscopic polymer chains are packed together.
Density Values of Common Plastics
The density of a plastic is fundamentally determined by its molecular structure, specifically the efficiency of polymer chain packing, which chemists can control during manufacturing. Polymers are broadly classified as either amorphous, having a random molecular structure and lower density, or crystalline, having a systematic and compact arrangement that results in higher density. The density values for plastics span a range, but they are most often compared to the density of water, which is approximately 1.0 g/cm³ at standard conditions.
Low-Density Plastics (Float)
Low-density plastics have values below 1.0 g/cm³ and are buoyant in water. These materials are often chosen when flexibility and lightweight properties are required.
- Polypropylene (PP): Commonly used in bottle caps and durable containers, its density is around 0.905 g/cm³.
- Low-density polyethylene (LDPE): Found in plastic bags and flexible containers, its density ranges from 0.91 to 0.93 g/cm³.
- High-density polyethylene (HDPE): Used for milk jugs and detergent bottles, it is slightly denser, falling between 0.94 and 0.97 g/cm³, but still remains buoyant.
High-Density Plastics (Sink)
High-density plastics have values above 1.0 g/cm³ and will sink in water. These materials typically offer greater structural strength and rigidity.
- Polyethylene terephthalate (PET): The material in most single-use beverage bottles, its density is approximately 1.38 to 1.39 g/cm³.
- Polyvinyl chloride (PVC): Often used for pipes and window frames, its density ranges from 1.16 to 1.45 g/cm³, depending on additives.
- Polystyrene (PS): Used in cutlery and solid foam containers, its density is typically around 1.04 to 1.09 g/cm³.
Density’s Role in Sorting and Application
The wide variation in plastic density is crucial for industrial sorting and material application. In recycling, density is the basis for sink-float separation, an effective method for separating mixed plastics. This process involves placing shredded plastic waste into a fluid, often water, where low-density plastics float and high-density plastics sink. This allows recyclers to quickly separate materials like floating HDPE from sinking PET.
For materials with similar densities, such as polypropylene and polyethylene, specialized liquids with a density between the two plastics may be used to achieve finer separation. The ability to separate polymer types based on density is a foundational step in ensuring the purity of recycled plastic resins, which directly influences the quality and value of the final recycled product.
Density also dictates a plastic’s suitability for various products. Low-density polymers, such as LDPE, are favored when flexibility and lightweight properties are required, making them ideal for packaging films and bags. Higher-density materials, like HDPE and PVC, offer greater structural strength and rigidity, making them the preferred choice for durable items such as pipes and automotive components.