What is polystyrene box (EPS)? Material and insulation explained
Short answer: Polystyrene box is the common name for EPS, expanded polystyrene. In Germany it is called Styropor, a trademark. The material is a plastic and consists of approximately 98% air. The air enclosed in the cells provides the insulation. That's why polystyrene boxes keep products so well cooled.
In this article, you will learn what the material is made of, how it differs from XPS (Styrodur) and which values are common for density and thermal conductivity. Then you will see what that means in practice for a polystyrene box.
What is a polystyrene box?
Styropor was originally a trademark of BASF. According to Wikipedia, the German industry organisation Industrieverband Hartschaum (IVH) has managed the trademark since the nineties. However, the name has become a common word, like Tempo for paper tissues.
The technical term is EPS, expanded polystyrene. Polystyrene box, Styropor and EPS mean the same thing in everyday use. We therefore use both terms alongside each other.
What is polystyrene box made of?
Polystyrene box is made of polystyrene. After expansion, according to the IVH guide, it consists of approximately 98% air. The solid part is therefore very small.
The material is created in three steps:
- Beads. You start with small polystyrene beads containing a propellant gas. Pentane, cyclopentane or carbon dioxide are often mentioned.
- Expansion. With heat, usually steam, the beads expand to a multiple of their original volume.
- Welding. In the mould, the expanded beads melt together into blocks, plates or moulded parts such as a box.
At the break edge you can see the beads again. EPS looks like many beads baked together.
Is polystyrene box plastic?
Yes. Polystyrene is a plastic. Polystyrene box is therefore plastic in expanded form. How to dispose of it, you read in disposal and recycling of polystyrene box.
Polystyrene box, EPS and Styrodur: the differences
The terms are often used interchangeably. Styropor and Styrodur are both BASF trademarks, but for two different foams.
| Term | Means | Production | Appearance |
|---|---|---|---|
| Styropor | Trademark for EPS | Beads expanded and welded together in the mould | White or grey |
| EPS | Expanded polystyrene (technical term) | Like Styropor | White or grey |
| Styrodur | Trademark for XPS (extruded polystyrene) | Polystyrene melted, expanded and extruded through a nozzle | Green at BASF, also blue or pink at other manufacturers |
XPS is denser and absorbs less water. Baustoffshop states for XPS with long-term submersion only approximately 0.7 vol.%. It is unusual for packaging. For cooling boxes, EPS is used because it is light, easy to shape and insulates well.
Properties of EPS in short
Density
The density of EPS varies greatly. According to Wikipedia, it ranges from approximately 15 kg/m³ for insulation in construction to 90 kg/m³ for products such as a ski helmet. The higher the density, the stiffer but also heavier the part. For insulation, both density and wall thickness matter.
Thermal conductivity
The thermal conductivity coefficient (lambda) shows how much heat a substance conducts. The lower the value, the better the material insulates.
| Material | Thermal conductivity in W/(m·K) | Source |
|---|---|---|
| White EPS | 0.035 to 0.040 | Wikipedia, IVH guide |
| Grey EPS (with graphite) | approximately 0.032 | Wikipedia, IVH guide |
| XPS (Styrodur) | 0.033 to 0.035 | Baustoffshop |
These are values for building boards. For boxes, the effect also depends on wall thickness, lid and closure.
Temperature
EPS is suitable for continuous operation up to approximately 70 °C. According to Wikipedia, aging accelerates from 55 °C. Therefore, do not leave boxes in a hot car in the sun and keep them away from heat sources.
What is EPS used for?
You will find EPS in many places. Most well-known are insulation boards for facades, walls and roofs. Then there are packagings, such as moulded parts around electronics, filling in packages and boxes for cooled products. Requirements differ: in construction, insulation value and fire safety matter, in packaging, weight, shock protection and temperature retention. Therefore, density and shape vary by product.
Why do polystyrene boxes insulate so well?
Heat transfers in three ways: conduction, convection and radiation. In EPS, air is in small, closed cells. It can barely flow and conducts heat poorly. The material itself is only a small part. Thus the cold stays in the box and heat from outside enters only slowly.
In practice, wall thickness decides. In our range you will find polystyrene boxes with wall thicknesses from 15 to 50 mm. Reference values from our practical tests at approximately 20 °C outside temperature:
| Construction | Cooling time (reference value) |
|---|---|
| Thin wall (20 mm) with 2 gel packs | 8 to 12 hours |
| Thick wall (40 mm) with 2 gel packs | 24 to 36 hours |
| Thick wall (40 mm) with 4 gel packs | 36 to 48 hours |
| Thick wall (50 mm) with dry ice | 48 to 72 hours (frozen) |
These are experience values, not a guarantee. Outside temperature, filling level and pre-cooling affect the result. Read more in how long does a polystyrene box stay cold.
Polystyrene box in the cold chain
For food and medicine, a constant temperature is important throughout the transport route. A box made of EPS weighs little, dampens shocks and can be combined with gel packs. For short shipments, a thin wall is often sufficient. For long routes or summer heat, you better choose a thicker wall. Which size fits, you read in choosing the right box size.
What to watch out for with a polystyrene box?
- Wall thickness. This determines how long the temperature lasts. Choose the wall appropriate for the transport time.
- Internal dimensions instead of external dimensions. What really fits counts. A box of 4.7 litres with 30 mm wall is 300 x 200 x 200 mm on the outside, but only 240 x 140 x 140 mm on the inside.
- Drainage holes. Boxes without holes are also suitable for liquids. Boxes with holes let melted cooling gel drain away.
- Cardboard box. For shipping and stacking, a suitable cardboard box protects against impacts.
- Cooling medium. Take gel packs or ice packs along directly.
Does polystyrene box burn? Is polystyrene box flammable?
Yes, polystyrene box is flammable. According to EN 13501-1, EPS falls into European fire class E. As an insulation material, EPS is equipped with a flame retardant. According to the IVH guide, such insulation material is considered difficult to ignite (B1 according to DIN 4102).
Whether a particular packaging contains a flame retardant depends on the product. Check the product data sheet or ask us. In practice: Keep polystyrene boxes away from open flames, cigarettes, space heaters and welding. Do not burn residues in the stove or garden.
Frequently asked questions about polystyrene box and EPS
What is the difference between Styropor and EPS?
Styropor is a trademark, EPS is the technical term. Both mean expanded polystyrene. In everyday use, they mean the same thing.
Is Styropor the same as Styrodur?
No. Styropor is EPS, Styrodur is XPS. The two foams are made differently. XPS is denser and absorbs less water.
Is polystyrene box plastic?
Yes. Polystyrene box consists of polystyrene, a plastic, and approximately 98% air.
How well does polystyrene box insulate?
White EPS achieves a thermal conductivity of approximately 0.035 to 0.040 W/(m·K), grey EPS approximately 0.032. A lower value means better insulation.
Is polystyrene box flammable?
Yes. Keep it away from open flames. Insulation material with flame retardant is considered difficult to ignite, packagings do not need to be.
Why do polystyrene boxes keep products cool for so long?
Air in closed cells conducts heat poorly. With a thick wall and gel packs, the temperature remains stable for many hours.
Find the right polystyrene box
Do you need boxes for cooled shipments? In our range buy polystyrene boxes you will find many sizes and wall thicknesses, manufactured by us since 1972. Looking for a reusable alternative? Read choosing between EPS and EPP and check out EPP boxes.
