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 industry, the sponge next to your kitchen sink may seem similar to the foam mattress pad on your bed. After all, both are soft and compressible. 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.

What’s different about foam rubber? And how does it compare to sponge rubber? The differences begin with ingredients and end with molecular structure. Foam rubber and sponge rubber also use different raw materials, chemical reactions, and production processes. Not all foam rubber is soft like your mattress pad either, and that enables applications for rigid foams in building and construction.  

How Foam Rubber Is Made

Foam rubber uses a blowing agent, typically a gas or a chemical that produces a gas, to create a mass of small bubbles in a liquid mixture. This mixture may contain polyols, polyisocyanates, water, and additives such as flame retardants, fillers, and colorants. There are many different types of blowing agents that are capable of producing a cellular structure, and the compounder controls foaming by adjusting the amount of water or by using surfactants.

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 foam rubber that is either flexible or rigid. During polymerization, molecules from the polyols and polyisocyanates crosslink to form three-dimensional structures.

The importance of blowing agents in the production of foam rubber cannot be overstated. 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 seems similar to foam rubber, but the two are 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.

Get Help Choosing Foam Rubber vs. Sponge Rubber

Do you need help with material selection for your next engineering project? Elasto Proxy can help you decide whether foam rubber or sponge rubber is the right choice for your application. We can also explain the differences between these two options and solid rubber materials. As an experienced fabricator of seals, gaskets, and insulation, we can also help you with compound selection.

To get started, contact us.  

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