Polyethylene Terephthalate, commonly known as PET, is a thermoplastic polymer resin ubiquitous in modern packaging and textiles. It is a polyester material recognized for its strength, lightness, and clarity, making it a preferred choice for containers like plastic bottles and various food packages. Density, a measure of mass divided by volume, is instrumental in determining PET’s performance characteristics and how it is recycled.
The Standard Density of PET
The density of Polyethylene Terephthalate is consistently greater than that of water, which has a density of 1.0 gram per cubic centimeter (g/cm³). The typical density range for PET falls between 1.33 and 1.45 g/cm³. This high density means that PET contains significantly more mass than the same volume of water, causing it to sink when placed in water.
The most common density for PET used in soft drink and water bottles is cited around 1.37 to 1.39 g/cm³. This relatively high density contributes to the material’s strength and structural integrity, which is necessary for containing pressurized liquids like carbonated beverages. The density is not a fixed number but rather a range that changes based on how the polymer is processed during manufacturing.
How Crystallinity Affects PET Density
The reason PET density exists as a range is due to the degree of crystallinity, which describes how organized the polymer chains are at a molecular level. PET exists in two main states: amorphous, where the chains are disordered, and crystalline, where they are tightly packed in an orderly structure. The manufacturing process, particularly the speed of cooling, dictates the ratio of these two states, influencing the final density.
Amorphous PET forms when the molten polymer is cooled rapidly, preventing the molecular chains from fully aligning, resulting in a lower density, typically around 1.33 g/cm³. This lower-density form is transparent and is used for thin-walled applications like beverage bottles where clarity is important.
Conversely, when the polymer is cooled slowly or reheated, the chains organize into denser crystalline regions, reaching up to 1.45 g/cm³. This higher-density, crystalline PET is more opaque, exhibiting greater strength and thermal resistance, making it suitable for engineering applications or thicker containers.
Density and Common Applications of PET
The relatively high density of PET compared to other packaging plastics contributes to its rigidity and high tensile strength. This allows PET to be used in lightweight containers without compromising the product’s integrity.
The structure that creates this density also provides excellent barrier properties, resisting the passage of gases like carbon dioxide. This gas retention capability is why PET is chosen for carbonated beverage bottles over lower-density plastics like polyethylene or polypropylene. The compact molecular structure limits the ability of gases to permeate the material, keeping drinks fresh and carbonated for longer periods.
Density’s Role in Plastic Sorting and Recycling
PET’s density, which is greater than 1.0 g/cm³, is fundamental to its recycling process. Recycling facilities use a technique called ‘sink-float separation’ to purify the shredded plastic stream. When granulated plastics are introduced into water, the heavier PET flakes sink to the bottom of the separation tank.
This sinking behavior allows for the effective separation of PET from common contaminants, such as bottle caps and labels. These are often made from lighter plastics like Polypropylene (PP) or High-Density Polyethylene (HDPE). Since PP and HDPE have densities less than 1.0 g/cm³, they float on the water’s surface, while the PET collects at the bottom. This density-based sorting ensures a high-purity stream of recycled PET, which is necessary for manufacturing new plastic products.