Foam Rubber vs. Sponge Rubber: What’s the Difference?

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Did you know that there's a difference between foam rubber and sponge rubber? Sure, the two terms are often used interchangeably. Unless you work in the rubber and plastics industry, the sponge next to your kitchen sink may seem similar enough to the foam mattress pad on your bed. Both substances are soft and squishy, right? Upon closer examination, however, saying that foams are identical to sponges is like saying that doing the dishes is the same as getting a good night's sleep.

For technical buyers, choosing the right material is a lot more important than finding the right analogy. The memory foam from a mattress might help with after-dinner cleanups, but a polymer kitchen sponge is a better choice. For safety-related applications, selecting the right rubber material may also mean meeting requirements for flame, smoke, and toxicity (FST). In the mass transit industry, for example, some silicone foams meet FST standards but many carbon black foams do not.

What's so different about these foams, and how does foam rubber compare to sponge rubber anyway? Let's take a look at how these polymers are made, and consider how raw materials, chemical reactions, and production processes can affect the characteristics of foam and sponge rubber.

How Foam Rubber Is Made

The differences between foam and sponge rubber begin with ingredients and end with molecular structure. Foam rubbers use a blowing agent, typically a gas or a chemical that produces a gas, to create a mass of small bubbles in a liquid mixture. Typically, this mixture contains polyols, polyisocyanates, water, and chemicals or additives such as flame retardants, fillers, and colorants. There are many different types of blowing agents, and foaming is used for both rubber molding and rubber extrusion.

The polyols and polyisocyanates in foam rubber are liquid polymers that, when combined with water, produce a heat-generating or exothermic reaction. By using specific types and combinations of liquid polymers, a material compounder can create flexible or rigid foam rubbers. During polymerization, molecules from the polyols and polyisocyanates crosslink to form three-dimensional structures. The compounder can control foaming by adjusting the amount of water, or by using surfactants.

The importance of blowing agents in the production of foam rubber cannot be overstated. Although you can do the dishes without a sponge and get a good night's rest without a mattress pad, a compounder cannot create foam rubber without a blowing agent. Typically, flexible foams use the carbon dioxide gas formed by the reaction of water with the polyisocynate. Most rigid foams use hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs), gases with higher levels of toxicity and flammability than found in chlorofluorocarbons (CFCs).

How Sponge Rubber Is Made

Sponge rubber may be similar to foam rubber, but the two are not one and the same. For starters, there are two main types of sponge rubber: open-cell and closed-cell. Open-cell sponge rubber contains open, interconnected pockets that permit the passage of air, water, and other chemicals when the material is not compressed. Closed-cell sponge rubber contains balloon-like cells that hold nitrogen gas and thus prevent the passage of these substances at low pressures.

To produce open-cell sponge rubber, sodium bicarbonate is added to other ingredients in a heated mold. As the uncured sponge rises like a cake, the baking soda creates open, interconnected cells. To make closed-cell sponge rubber, a chemical powder that decomposes under heat and pressure is added. The nitrogen gas that's released helps to give closed-cell sponge rubber its strong compression set and recovery characteristics.

Although nitrogen is a gas, it doesn't produce a foam like the gaseous blowing agents used with foam rubber. Foaming is specific production process, and foam rubber contains mostly open cells. Although some of the cells in foam rubber are closed, these rubber materials would not pass ASTM tests for water absorption, a standard requirement for closed-cell materials.

About the Author: Doug Sharpe is the President of Elasto Proxy, Inc. (Boisbriand, Quebec, Canada), supplier of sealing solutions and custom-fabricated rubber and plastic parts to a variety of industries, including green power, automotive, aerospace, and defense.