Expanded Polystyrene (EPS Foam): Uses, Structure & Properties

What is expanded polystyrene (EPS)?

Expanded PolyStyrene (EPS) is a white foam plastic material. It is produced from solid beads of polystyrene. It is used for packaging and construction. It is a closed-cell, rigid foam material produced from:

Styrene – which forms the cellular structure
Pentane – which is used as a blowing agent

Both styrene and pentane are hydrocarbon compounds. They are obtained from petroleum and natural gas byproducts.

EPS is 98% air and it is recyclable. It is a very lightweight material with the following benefits:

It has very low thermal conductivity, low moisture absorption, and excellent cushioning properties.
Its physical properties do not change within its service temperature range (i.e., up to 167°F/75°C) for long-term temperature exposure.
Its chemical resistance is nearly equivalent to the material upon which it is based – polystyrene.

One of the serious limitations of polystyrene foam is its low maximum operating temperature of ~80°C.

How is expanded polystyrene made?

The conversion of expandable polystyrene to expanded polystyrene is carried out in three stages:

Pre-expansion,
Maturing/Stabilization, and
Molding

Polystyrene is produced from crude oil refinery product styrene. For manufacturing EPS, the polystyrene beads are impregnated with the foaming agent pentane. Polystyrene granulate is prefoamed at temperatures above 90°C.

This temperature causes the foaming agent to evaporate. Hence inflating the thermoplastic base material to 20-50 times its original size.

After this, the beads are stored for 6-12 hrs allowing them to reach equilibrium. Then beads are conveyed to the mold to produce forms suited as per application.

Manufacturing Of Expanded Polystyrene Sheets/Molds

During the final stage, the stabilized beads are molded in:

either large blocks (Block Molding Process)
or designed in custom shapes (Shape Molding Process).

The material can be modified by the addition of additives such as flame retardant to further enhance the fire behavior of EPS.

What are the benefits of EPS?

EPS is a lightweight material with good insulation characteristics offering benefits such as:

Thermal Properties (insulation) – EPS has very low thermal conductivity. This is due to its closed-cell structure consisting of 98% air. This air trapped within the cells is a very poor heat conductor. Hence provides the foam with excellent thermal insulation properties. The thermal conductivity of expanded polystyrene foam of density 20 kg/m³ is 0.035 – 0.037 W/(m·K) at 10°C.

ASTM C578 is a Standard Specification for Rigid Cellular Polystyrene Thermal Insulation. It addresses the physical properties and performance characteristics of EPS foam as it relates to thermal insulation in construction applications.

Mechanical strength – Flexible production makes EPS versatile in strength. This strength can be adjusted to suit specific applications. EPS with high compressive strength is used for heavy load-bearing applications. For void-forming EPS, lower compressive strength can be used.

Generally, strength characteristics increase with density. However, the cushioning characteristics of EPS foam packaging are affected by the geometry of the molded part and, to a lesser extent, by bead size and processing conditions, as well as density.

Density (pcf)	Stress @ 10% Compression (psi)	Flexural Strength (psi)	Tensile Strength (psi)	Shear Strength (psi)
1.0	13	29	31	31
1.5	24	43	51	53
2.0	30	58	62	70
2.5	42	75	74	92
3.0	64	88	88	118
3.3	67	105	98	140
4.0	80	125	108	175

Typical Properties of EPS Molding Packaging (70°F Test Temperature)

(Souce: EPS Industry Alliance)

Dimensional Stability – EPS offers exceptional dimensional stability. It remains virtually unaffected by a wide range of ambient factors. The maximum dimensional change of EPS foam can be expected to be less than 2%. This puts EPS in accordance with ASTM Test Method D2126.

Electrical Properties – The dielectric strength of EPS is approximately 2KV/mm. Its dielectric constant is measured in the frequency range of 100-400 MHZ. At gross densities from 20-40 kg/m³ the dielectric constant lies between 1.02-1.04. Molded EPS can be treated with antistatic agents to comply with electronic industry and military packaging specifications.

Water Absorption – EPS is not hygroscopic. Even when immersed in water it absorbs only a small amount of water. As the cell walls are waterproof, water can only penetrate the foam through the tiny channels between the fused beads. View all EPS grades with low-none water absorption »

Chemical Resistance – Water and aqueous solutions of salts and alkalis do not affect expanded polystyrene. However, EPS is readily attacked by organic solvents. Check out EPS grades with good chemical resistance »

Weathering and Aging Resistance – EPS is resistant to aging. However, exposure to direct sunshine (ultraviolet radiation) leads to a yellowing of the surface. This is accompanied by slight embrittlement of the upper layer. Yellowing has no significance for the mechanical strength of insulation, because of the low depth of penetration.

Fire Resistance – EPS is flammable. Modification with flame retardants significantly minimizes the ignitability of the foam and the spread of flames.

Expanded Polystyrene Properties and Key Benefits

What is the difference between EPS vs XPS?

XPS is often confused with EPS. EPS (expanded) and XPS (extruded) are both closed-cell rigid insulation made from the same base polystyrene resins. However, the difference lies in their manufacturing process.

Expanded Polystyrene (EPS)

Extruded Polystyrene (XPS)

EPS is manufactured by expanding spherical beads in a mold, using heat and pressure to fuse the beads together. While each individual bead is a closed-cell environment, there are significant open spaces between each bead

EPS beads are molded in large blocks that are subsequently cut by hot-wire machines into sheets or any special shape or form by computer-driven systems

EPS’s blowing agent leaves the beads rather quickly creating thousands of tiny cells full of air

EPS absorbs more water than XPS resulting in reduced performance and lost insulation power (R-value)

XPS is manufactured in a continuous extrusion process that produces a homogeneous "closed cell" matrix with each cell fully enclosed by polystyrene walls

XPS is "extruded" into sheets. Polystyrene is mixed with additives and a blowing agent – which is then melted together through a dye

XPS’s blowing agent stays embedded in the material for years

XPS is often selected over EPS for wetter environments that require a higher water vapor diffusion resistance value

The compressive strength of XPS is greater than that of EPS

Source: Owens Corning

Is EPS recyclable?

EPS Insulation is composed of organic elements – carbon, hydrogen and oxygen – and does not contain chlorofluorocarbons (CFCs) or hydrochlorofluorocarbons (HCFCs). EPS is recyclable at many stages of its life cycle.

Expanded Polystyrene is 100% recyclable and is designated by plastic resin identification code 6. View Recycled EPS Grades »

However, the collection of EPS can be a major challenge as the product is very light. PS recyclers have created a collection system in which the EPS is shipped over short distances to a facility where the material is further processed by:

Granulation – EPS is added into a granulator that chops the material into smaller pieces.
Blending – the material is passed into a blender for thorough mixing with similar granules.
Extrusion – the material is fed into the extruder, where it is melted. Color can be added, and the extruded material is then molded into a new value-added product.

EPS materials can be reprocessed and molded into new packaging products or durable goods

Several countries have established formal expanded polystyrene recycling programs throughout the world.

Check Out an Interesting Video on Expanded Polystyrene Recycling!
Source: Moore Recycling Associates

Is EPS toxic?

EPS is inert and non-toxic. It does not leach any substances into the groundwater. It is safe to use for diverse packaging applications. It does not release harmful chemicals into the environment.

What are the sustainability benefits of EPS?

Sustainability benefits associated with EPS are:

Energy efficiency – It aids energy savings as it is an effecting thermal insulation material which helps reduce CO₂ emissions.

Recyclability – EPS is recyclable at many stages of its life cycle. No residual solid waste is generated during its manufacturing.

Lightweight – EPS is lightweight. It reduces transportation costs, fuel consumption, and greenhouse gas emissions. They provide a positive impact on the environment.

Durability – EPS is a durable material. The products made from EPS can be reused or repurposed for other applications.

Non-toxic – EPS is considered safe to use in food packaging. It does not release harmful chemicals into the environment.

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Expanded Polystyrene (EPS): Ultimate Guide on Foam Insulation Material