1. Expanded Polystyrene Is About 98% Air by Volume
According to the Expanded Polystyrene Association (JEPSA), expanded polystyrene products consist of approximately 98% air by volume, with the raw material making up only 2%.[1] This figure is also confirmed in the theme materials for the Tokyo Business Design Award, which state that 98% is air and only 2% is plastic raw material.[3]
It is important to note that this 98% refers to volume, not weight. It does not mean that 98% of the weight is air. The reason a large box feels light is that most of the space it occupies is filled with air, while the amount of resin is very small.
2. How 1mm Polystyrene Beads Are Expanded with Steam and Molded
The raw material for expanded polystyrene consists of polystyrene beads (raw beads) with a diameter of about 1mm.[2] The manufacturing process happens in two stages. First, steam is applied to these beads to make them expand. This step is called pre-expansion, and in many products, the volume increases to about 50 times its original size.[2]
Next, these expanded beads (expanded beads) are placed into a metal mold, and steam is applied again. The beads stick together, forming a box or sheet in the shape of the mold.[2] You can imagine this as blowing up many balloons and pressing them together inside a box until they stick. However, unlike balloons, air is not blown in from the outside; the beads themselves expand due to the steam.
The "50 times" figure is a rough guide, and the source notes that products can be made with other expansion ratios.[2] The size of the raw beads is also described with some variation on the association’s page, listed as both "about 1mm" and "0.3 to 2mm."[2]
3. Checking the "50 Times" and "98% Air" Figures with a Simple Calculation
You can check if the association’s figures of "about 50 times" and "98% air" match by doing a simple calculation. Even when the beads expand, the amount of resin does not change. If the volume becomes 50 times larger, the proportion of resin becomes 1/50. Calculating 1 ÷ 50 equals 0.02, which is 2%. The remaining 98% is air.[1]
You can also estimate the size of the beads. If the volume increases by 50 times, the length does not increase by 50 times. If you assume the beads remain spherical and expand evenly, the diameter increases by the cube root of 50, which is about 3.7 times. So, a 1mm bead would become about 3.7mm. This is just a rough estimate because the actual shape of the beads is irregular.
Note: If the mass of the resin stays the same but the volume becomes k times larger, the volume ratio of the resin becomes 1/k.
4. What the Closed Cells of Trapped Air Do Besides Making It Light
It is not just that air makes up most of the volume; it is crucial that this air is trapped in small rooms called closed cells.[1] The expanded beads connect like a honeycomb, with each individual room holding air.[4]
Air does not conduct heat well, so when these small rooms line up, heat transfer is reduced. When the material is hit, the air in the small rooms acts as a cushion to absorb the impact.[4] Lightness, insulation, and cushioning are not separate properties but are created by the same small rooms.[1]
This property made the material suitable for fish boxes. Around 1967, expanded polystyrene fish boxes with holes began replacing wooden boxes, allowing fresh fish to be transported to consumption areas with ice and seawater.[6] The insulation provided by the air in the small rooms was well-suited for this purpose.
5. How Expanded Polystyrene Started as a Substitute for Cork
Although expanded polystyrene is strongly associated with fish boxes today, it is introduced as having started as a substitute for cork. It was developed in Germany in 1950 and is said to have been produced domestically in Japan around the late 1950s.[6]
The BASF chronology notes that the production of Styropor began in 1951. This material is 98% air by volume and has excellent insulation properties. The chronology also states that the first experiments were conducted in a shoe polish can.[5] Lightness and insulation are introduced as characteristics from the beginning.
The material born as a cork substitute later spread as fish boxes and insulation for houses. The same small air rooms have found different jobs depending on the era.
6. How to See the Tiny Beads and Air Cells in a Broken Piece at Home
If you have an expanded polystyrene box or packaging material at home, look at a part that is already broken or missing using a magnifying glass. You should see traces of small beads stuck together. Use a ruler to measure the size of the beads and think about how many times larger they look compared to the 1mm raw material.
Safety note: For safety, do not put pieces in your mouth, do not tear them by hand to scatter them, and do not do this near small children. Do not break new pieces; use surfaces that are already broken.
After looking, search for places in your home where expanded polystyrene is used. In those places, are the small air rooms being used for lightness, heat retention, or cushioning? Thinking about the reasons will reveal different faces of the same material.
Safety note: For safety, do not break new pieces, and do not put pieces in your mouth.