The difference between foam rubber vs. sponge rubber begins with ingredients, continues through the material manufacturing process, and extends to molecular structure. Although the terms “foam rubber” and “sponge rubber” are sometimes used interchangeably, these elastomers have differences that may not be readily apparent. Moreover, sponge rubber and foam rubber are used in different types of sealing and insulation applications.
If you don’t understand how foam rubber vs. sponge rubber are made and used, you risk choosing a material that permits leaking, provides inadequate cushioning, or that won’t withstand the environment. The rubber may also shrivel up, become brittle, or lose compressibility. By choosing a compound based on MTAP, an abbreviation for material, temperature, application and pressure, engineers can meet requirements beyond whether just foam rubber vs. sponge rubber needs to be used.
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 because of their relationship to flexibility and rigidity. 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 Foam Rubber is Used
Flexible polyurethane foams are used for vibration control and shock absorption. They provide increased energy absorption with increased density, predictable performance under compressive force, and low permanent deformation under constant resistance. These foams won’t become wider when compressed, which makes them a good choice when space is tight. Applications include hood gaskets in mobile equipment, shock absorbers for industrial machinery, and vibration isolators for appliances.
Polystyrene foams are rigid, structural materials that are strong, lightweight and moisture resistant. They help reduce product weight and have a high stiffness-to-weight ratio. Other types of structural foams have a sandwich-like structure with a foam core between two thin but solid layers. Reticulated foams are used for filters and be compounded with bactericides, fungicides and other additives. Foam rubber that is fabricated into elastomeric pads can be attached to vacuum tooling for manufacturing.
During foam rubber fabrication, sheet materials or extrusions are converted into finished products. Water jet cutting makes fine, fast cuts and eliminates the mis-cuts and material waste associated with manual cutting operations. Custom-fabricated foam rubber products also support the use of tapes that use either a heat-activated taping system (HATS) or a pressure-sensitive adhesive (PSA). A variety of bonding methods are available for custom gaskets, but not all of them are suitable for foam rubber.
How Sponge Rubber Is Made
Like foam rubber, sponge rubber has a cellular structure and is available in different densities. Typically, they are listed as soft, medium, and firm. There are two main types of sponge rubber. Open-cell materials contain 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.
How Sponge Rubber Is Used
Sponge rubber is made of neoprene, EPDM, nitrile, silicone, and many other elastomeric materials. Often, sponge rubber profiles are fabricated into finished gaskets that are used for shock absorption and provide good compression and recovery. Sponge rubber sheets also support custom fabrication, included value-added operations such as gasket taping. Compared to solid rubber, sponge rubber is softer and less resistant to compression; however, sponge rubber still has a high strength-to-weight ratio.
Open-cell foams are used in prosthetic devices, medical sponges, electrocardiogram (ECD) pads, medical filters, and sterilization bags. All of these applications require elastomeric components that allow the passage of water and gases. Foam rubber parts are also used in patient lifts, hospital room equipment that helps people with limited mobility to sit up or get up. Closed cell sponge rubber that is made from fluorosilicone is used with pharmaceutical equipment such as tableting machines.
FDA approved silicone sponge rubber may be required for food contact or medical applications. There’s a different between FDA approved and FDA compliant, however, so buyers need to perform due diligence during material selection. Sponge rubber is also used in bulb seals for doors, hatches, and enclosures. These trim seals have separate bulb and retainer sections and are made of different materials. Typically, the bulb portion is made of an EPDM sponge rubber.
Get Help with Material Selection and More
Do you need help with material selection for your next engineering project? Elasto Proxy can help you decide whether foam rubber vs. 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 material selection and more.
To get started, contact us.