Open-Cell vs Closed-Cell Foam: Which do You Need?

Duncan Geddes

by Duncan Geddes

Which kind of foam is the right one for your needs? This article explains the properties and benefits of open-cell foam and closed-cell foam and how to tell them apart.

The first synthetic foam was manufactured in the late 1920s by Dunlop. The company made foam rubber by whipping air into natural latex. In the 1950s, US chemist Charles C Price invented polyether polyurethane foam, and today, the majority of foams are made from polyurethane.

Polyurethane is a polymer – a material comprising recurring units of hydrogen-carbon compounds. An example of a natural polymer is cellulose, which is found in the cell walls of most plants. Polyurethane is a thermosetting plastic; when it’s cured, a cross-linked polymer network is created, resulting in a strong, durable material.

Within the structure of foam, there are thousands of gas pockets, or pores, ranging in size from 10 pores per linear inch (PPI), meaning that each pore has a diameter of 0.1in (2.54mm), to 150 PPI, making each pore just 0.007in (0.178mm) across.

The suitability of a foam for a particular purpose depends largely on the size and structure of its pores, or ‘cells’.

What is closed-cell foam?

Closed-cell foam is foam that has tiny, discrete pockets of gas – each one totally enclosed within polymer walls. It is made by incorporating a blowing agent into the chemical reaction between liquid polymers used to make polyurethane. The material hardens, and the result is a strong closed-cell foam with a density of up to 800kg/m3 – ideal for insulation.

High-density, closed-cell foam will last for many years. As cavity-wall insulation, the longevity of this material can equal that of the building.

As an alternative to pre-formed foam slabs, insulation can be installed in the form of high-density expanding foam, which is applied in liquid form via a hand-held applicator-gun. The insulating properties of expanding foam (or ‘spray foam’) are exceptional, as the liquid polyurethane works its way into every crevice, foaming in reaction with the blowing agent. However, this super-efficient product can lead to restricted ventilation, which, in turn, can cause condensation and associated problems, like mould.

What is open-cell foam?

Open-cell foam is made by introducing variations in the blowing agent when making polyurethane. Reactions cause pockets of gas to expand and ‘open’. Even if only half of a foam’s pores are open, it’s still classed as open-cell foam to distinguish it from the tough, impervious, more expensive closed-cell foam.

One of the polymer composites of polyurethane is a diisocyanate, which reacts with water to form carbon dioxide. So, in the manufacture of a softer, open-cell polyurethane foam, water is added for the production of carbon dioxide gas.

With a density of around 15kg/m3 to 150kg/m3, this more forgiving type of foam is used in furniture and vehicle upholstery, mattresses, cleaning packaging. Open-cell foam is also used as insulation where a bit of air flow is desirable.

How can you tell if cell foam is open or closed?

The main difference between the different types of foam is that open-cell foam is softer and has more ‘spring’ while closed-cell foam is firmer. Depending on the amount of use it gets, open-cell foam won’t last as long as closed-cell foam. With less resistance to compression, the cells will break down over time and lose their spring. The thicker the foam, the sooner this will happen.

Is reticulated foam open-cell or closed-cell?

Reticulated foam is a variety of open-cell foam, created by removing the cell faces from partially closed-cell polyurethane foam. This sounds like an impossible task. But it can be done! The cells are three-dimensional polygons, the struts (or ‘edges’) more robust than the faces (or ‘planes’/’windows’/’sides’), which have a higher surface area in relation to their mass.

The partially closed-cell foam is filled with just the right amount of a combustible gas, such as hydrogen, oxygen, or a combination of the two. The gas is ignited, causing an explosion. The flimsy cell faces are destroyed, leaving the struts intact.

With the cell faces gone, the foam now has a different texture, with a density of as little as 15kg/m3. It has a greatly reduced resistance to compression, so it’s very soft and springy, and squashes easily. However, without the cell surfaces, the foam’s tensile properties increase, making it more malleable and less inclined to tear.

In this article, we’re focusing mainly on polyurethane foam. However, melamine foam, a polymer made from melamine (C3H6N6) and formaldehyde (CH2O), is certainly worth a mention. Melamine is a very hard polymer, and the struts have an abrasive quality, which is utilised in many cleaning products. Melamine’s high nitrogen content gives the material superb fire-resistance, and it’s therefore a popular choice for acoustic applications – a job melamine does very well.

What is R-value?

R-value is a measure used in the construction industry for thermal resistance per defined area. The higher the R-value, the better the insulation. Closed-cell foams have a much higher R-value than open-cell foams.

Should I use closed-cell or open-cell foam?

Closed-cell foam is mostly used for keeping stuff out – for example, as a barrier to moisture, heat, and sound. In contrast, open-cell foam is better for absorption.

Most of the world’s refrigerators and freezers are insulated by closed-pore polyurethane, and in the construction industry, this kind of foam is installed in cavity walls; it even contributes to the stability and load-bearing strength of a wall.

Open-cell reticulated foam performs well as a filter in aquaria, humidity pad or dust filter in air conditioners and oil filter in motor engines. Open-cell foam is also good at absorbing sound, improving acoustics in environments where echo is a problem. And it also makes a nice soft bath sponge!

Bespoke foam solutions

At Technical Foam Services, we don’t just supply foam. Our service is very much geared to providing innovative solutions, tailored to the individual needs of our customers. Need some inspiration? Take a look at our case studies.

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