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Open vs. Closed Cell Foam: Understanding Permeability

Posted by Polymer Technologies on 10/12/16 1:21 PM

Porous plastic foam is on the cutting edge of insulation for vapor, heat, noise, and other elements. The two major variants of porous plastic foams are open and closed cell foams. Both types of foam are used in everyday products, but because of their structural differences, one type of foam may perform better than the other depending on the desired application.

Open vs. closed cell foam.Foam is created by dissolving gas under high pressure into a polymer while it is in a liquid state, causing the formation of thousands of tiny bubbles, or cells, in the polymer. In closed cell foam, the cells are like tiny air pockets piled together in a compact configuration, resembling inflated balloons nestled tightly together. Due to the closely packed cells, closed cell foam is vapor semi-impermeable, more rigid, able to withstand greater pressure, and is about 4 times denser than open cell foam.

Created using a similar process, open cell foam is designated to be vapor semi-permeable as the formation of the cells in the material are broken, rather than closed. Similar to the holes inside a sponge, air can permeate the open cells more easily, making open cell foam more porous and absorbent than closed cell foam.

Each foam has a distinct structure and permeability and performs differently depending on the application.The main difference that cause manufacturers to choose between open and closed cell materials are their permeability to different elements—meaning how effective they are as barriers.

Air Permeability

Closed cell foam is a better air barrier than open cell foam, and can be used to control airflow as it is less permeable. For example, closed cell foam could be an effective gasket or seal for climate-control by keeping hot outside air from entering an air conditioned enclosure.

Open cell foam, since it allows air to pass through it, is more effective for filtration than closed cell foam. For instance, open cell foam makes a suitable filter for an engine because it can capture dust and pollutants but not restrict air flow.

Water Vapor Permeability

When it comes to preventing water vapor from passing through, closed cell is more useful. Closed cell foam is more impermeable to water, vapor
, and air, therefore it is less likely to be structurally impacted by the effects associated with water damage: mold, mildew, rot, and bacteria.Water Permeability and Temperature.

Open cell foam has a higher possibility of absorbing water, which can lead to degraded performance, especially for thermal applications. While engineers do not necessarily strive for perfect vapor impermeability, the free flow of water can be detrimental to a structure and can trap water.

If the environment is moist, closed cell foam might be the better option to work with as it’s less likely to absorb water and become an ineffective insulator. For example, closed cell foam would be better suited for wrapping a water tank than open cell foam.  

Heat Insulation

Both open and closed cell foam are efficient thermal insulators. Depending on the application and environmental factors, however, one type of foam might perform better than the other, especially if the environment is moist. For example, open cell foam might not perform optimally for thermal applications in moist or humid environments: a wet sponge will not retain or deflect heat effectively as water is a poor insulator in comparison to air.

Noise Isolation

Open cell foam performs better at absorbing sound than closed cell due to its permeability. The open cell structure allows sound waves to interact with residual membranes so that the energy is transformed into heat, absorbing some of the sound. Due to its ability to absorb sound, open cell foam is sought after for acoustical applications more than closed cell.

Choosing between open and closed cell foam

When selecting a material, cost is often a factor that impacts the decision of engineers and manufacturers just as much as the properties of a particular foam. Open cell foam is considerably more economical; it is less expensive to achieve the same thermal insulation from open cell than closed cell foam because it uses less plastic to make and the air within the open cell foam is an effective insulator.

In a broad sense, closed cell foam is semi-impermeable and restricts the flow of air and is less water absorbent, while open cell foam is semi-permeable and allows air and water to pass through it. Depending on your situation, one may be a more effective barrier than the other. If you are having difficulty finding the right kind of the porous plastic foam for your application, consult an expert to assist you.

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Topics: closed cell foam, open cell foam