The material commonly known as Styrofoam is technically Expanded Polystyrene (EPS), a lightweight foam used extensively in packaging and construction. Density measures an object’s mass relative to its volume, describing how much material is packed into a given space. Since EPS is a manufactured product, its density is not a single, fixed value but a range precisely engineered for different commercial purposes. This variability means that packaging materials and construction materials, like those used for roadbed stabilization, have different density profiles.
Defining the Average Density Range
The density of commercial Expanded Polystyrene products typically ranges from about 10 kilograms per cubic meter (kg/m³) to 35 kg/m³. This translates to approximately 0.6 to 2.2 pounds per cubic foot. For comparison, water has a density of 1,000 kg/m³, highlighting the vast amount of air contained within the foam structure. Manufacturers classify products into specific grades, such as Standard Light, Medium, and High density, each with a controlled value. The lowest-density grades, around 13.5 kg/m³, are used for general packaging where minimal weight is the primary goal. Conversely, the higher end of the range, closer to 28 kg/m³, is reserved for applications demanding greater structural integrity.
The Science Behind Low Density
The low density of EPS results from its manufacturing process. The process begins with small, hard polystyrene beads containing a liquid hydrocarbon blowing agent, typically pentane. When exposed to steam, the heat softens the thermoplastic polystyrene, causing the pentane to vaporize. The gas pressure forces the softened plastic to expand significantly, often up to 40 times its original volume, creating a network of interconnected cells within each bead.
The expanded beads are then molded together using more steam, fusing them into a solid block or shape. This final structure consists of closed-cell pockets where the vast majority of the volume is trapped air, not plastic. Due to this cellular structure, the final EPS product is composed of approximately 95% to 98% trapped air by volume. This high proportion of air, which has negligible mass, accounts for the material’s low density.
Factors Influencing Styrofoam Density
The final density of a polystyrene foam product depends on the intended application and the manufacturing method used. Density is directly correlated with compressive strength; products needing to bear weight, such as insulation for concrete slabs, require higher density than simple cushioning materials. Manufacturing controls, including the amount of steam and the time allowed for expansion, are precisely managed to achieve the target density and associated strength. Adjusting the pre-expansion stage determines the final density of the beads before they are molded.
EPS vs. XPS Density
Expanded Polystyrene (EPS) differs from Extruded Polystyrene (XPS), which is made using a continuous extrusion process. XPS foam generally has a higher and more consistent density, typically ranging from 32 kg/m³ to 45 kg/m³. This is due to its tightly packed, uniform closed-cell structure. This increased density makes XPS a better choice for demanding, moisture-prone applications, while lower-density EPS remains the economical standard for packaging and general insulation.