Plastic is not a single material but a broad category of synthetic polymers, meaning there is no single fixed value for “plastic density.” These materials are long chains of molecules synthesized from various feedstocks, and their physical properties, including density, differ significantly based on their chemical structure and processing methods. Density is a fundamental material property, measuring the mass contained within a specific volume. Understanding a polymer’s specific density is central to material science, dictating its weight, strength, and how it can be separated from other materials in applications ranging from manufacturing to recycling.
Understanding Density and Specific Gravity
Density (\(\rho\)) is defined as the mass (\(m\)) of a substance divided by its volume (\(V\)), expressed by the formula \(\rho = m/V\). For plastics, this value is most commonly measured in grams per cubic centimeter (g/cm\(^3\)). Density indicates how tightly a substance’s matter is packed, influencing its weight for a given size.
Specific Gravity (SG), also known as Relative Density, is the standard way density is often expressed for polymers. SG is a dimensionless ratio comparing the plastic’s density to that of a reference substance, typically water. Because water’s density is approximately 1.0 g/cm\(^3\), the Specific Gravity of a plastic is numerically almost identical to its density in g/cm\(^3\). This relative measure is useful because any plastic with an SG greater than 1.0 will sink in water, while any plastic with an SG less than 1.0 will float.
The Wide Range of Plastic Densities
The reported density of a plastic is a range, not a single point, due to variations in the polymer’s internal structure and the inclusion of additives. The primary factor influencing density is the arrangement of polymer chains, specifically the degree of crystallinity. Crystalline regions feature tightly packed, ordered chains, resulting in higher density, while amorphous regions are more disorganized and less dense.
This variability is evident in polyethylene, which exists in low-density (LDPE) and high-density (HDPE) forms. LDPE has more branching in its chains, resulting in a lower density, typically ranging from 0.91 to 0.93 g/cm\(^3\). HDPE has fewer branches, allowing chains to pack more closely, leading to a higher density between 0.95 and 0.96 g/cm\(^3\). Both types of polyethylene float in water since their specific gravities are below 1.0.
Other common plastics are denser and sink in water because their specific gravities exceed 1.0. Polyvinyl Chloride (PVC) has a density range of 1.16 to 1.45 g/cm\(^3\), and Polyethylene Terephthalate (PET) is around 1.38 to 1.39 g/cm\(^3\). The inclusion of pigments, fillers, or reinforcing agents can further increase a plastic’s density, pushing it toward the higher end of its reported range. Overall, the density for the broader category of plastics can span from approximately 0.9 to over 2.2 g/cm\(^3\).
Applying Density in Manufacturing and Recycling
Density is a key consideration during material selection in manufacturing, especially when weight reduction is desired. Manufacturers select lower-density polymers like polypropylene (PP, approximately 0.90 g/cm\(^3\)) over higher-density options like PET for applications such as the automotive industry. Using a lower-density material allows for the production of more parts per pound of raw material, which impacts cost and yield. Density also affects performance, as higher-density polymers often offer greater strength and chemical resistance.
In plastic recycling, density is the property utilized in the Flotation Separation technique for sorting mixed plastics. This method involves grinding the mixed plastic waste into flakes and introducing them into a liquid medium, often water or a salt solution with an adjusted density. Plastics with a density lower than the liquid will float, while those with a higher density will sink, allowing for mechanical separation based on material type.
This sink-float process is highly effective for separating common “floaters” like HDPE and LDPE from “sinkers” such as PET and PVC. Recyclers can achieve more precise separation by adjusting the liquid’s density, for example, using a solution of 1.172 g/cm\(^3\) to separate intermediate densities. Efficient separation is necessary because even small amounts of contaminant plastic can degrade the mechanical properties of the recycled material, reducing strength and impacting crystallinity.
Determining Plastic Density
Determining the density of a solid plastic in a laboratory setting often relies on two main techniques. The first is the liquid displacement method, standardized by procedures such as ASTM D792, which applies Archimedes’ principle. This procedure involves weighing the plastic sample in the air and then weighing it again while fully immersed in a liquid of known density, such as distilled water. The difference in weight is used to calculate the sample’s volume, which then allows for the calculation of its density.
The Density Gradient Column technique (ASTM D1505) is used for precise measurements. This method uses a vertical column containing a mixture of two liquids, where the density gradually increases from top to bottom. The plastic sample is introduced, and its density is determined by the specific level at which the sample achieves neutral buoyancy. This technique is useful for measuring small samples or distinguishing between materials with very similar densities.