To say something is “made of foam” more than likely produces one of two mental images of the material; the white polystyrene seen in drinking cups, coolers and packing peanuts or the squishy, spongy material that makes up foam footballs and polyurethane sheet packaging. These are both correct assumptions, but as is true with many things, there is more than meets the eye with foam. The familiar, open-cell polyurethane foam is actually divided into two categories, ester-based polyurethane and ether-based polyurethane. If two samples were placed next to each other, the untrained eye would have difficulty deciphering any differences. But while similar enough to be leaves on the same branch of the urethane foam family tree, there are slight differences in these two unique materials that make each suitable for specific applications.
On a structural level, both materials are urethane polymers, which results in their similar appearances and textures. A very versatile compound, slight changes in additives to urethane polymers can result in the material forming hard plastics, soft foam products or anything between. While having some shared additives and some contrasting, the biggest differences in the two polyurethanes are their bases; polyether triol and polyester, in the ether and ester foams respectively. Through chemical reactions, these mixtures expand and eventually form the foam we are familiar with today.
Ester-based polyurethane is the older of the two formulations, being developed full-scale in Germany following World War II. The material was in early development stages before the war, but the growing need for resources limited testing and production. Ether-based polyurethane soon followed and, particularly through formulations that created hard plastics, revolutionized the textile world. Today, ether-based polyurethane is more prevalent in common foam products because the raw materials required to create it cost less than ester-based polyurethane. Ether-based foam also has better moisture resistance against hydrolysis, the breakdown of molecules in contact with water. However, ester-based polyurethane maintains unique qualities that surpass the performance of ether-based foam in some applications.
While still soft and compactible, ester-based polyurethane is slightly more rigid and supportive than its ether relative. This is a result of smaller cells being created during the forming process. While still open-celled, these tiny bubbles are more difficult to bend and flex than the larger ether-based cells. This produces a slightly better shock absorbing material out of the ester foam, which is one reason it is often used in charcoal packaging foam. That shock absorption, combined with a firm but supportive structure secures items in transit or storage while helping diffuse impact. Its excellent packaging qualities lend it to being treated with agents to create the pink Anti-Static foam that dissipates electro-static charges in sensitive electronic equipment or instruments. Because of its rigidity, ester-based polyurethane is also used for cleaning applications as sponges and mops. The tensile strength and durability of this product is also greater than its counterpart.
Ether-based polyurethanes on the other hand are more flexible, better in wet environments and more affordable to produce for the most part. Ether foams have larger cell structures than ester, which allow greater airflow and moisture permeability. This makes ether-based foams perfect for speaker foam, aquarium filter media* or air filter foam where air must be able to flow while still safeguarding delicate components. Ether-based foam is also developed into specially engineered fast-drying foam, featuring a very large cell structure that allows the flow of water through its quickly air-dried form. This fast-drying foam is particularly useful for marine cushions in boats or outdoor patio furniture cushioning where moisture has the potential to impact the products. Softer and smoother than most ester-based polyurethane, ether foams are more frequently used in situations where materials are in contact with their environment. Colored polyurethane foam sheets for acoustical foam are another example of this.
As is true with most things, a little knowledge can go a long way. Being aware of the differences between these two very similar materials can help you chose the product that is best for the job, not just better. In summation, this is a list of qualities and characteristics for both varieties of foam:
- Used Less Prevalently
- Rigid and Supportive
- Greater Tensile Strength
- Slightly Smaller Cells
- Used for Specific Jobs
- Susceptible to Hydrolysis
- Costlier to Manufacture
- Used More Prevalently
- Softer and Cushioning
- More Flexible
- Larger Cells
- Wider Range of Applications
- Resistant to Hydrolysis
- More Affordable
*NOTE: Always test filter media before installing in an environment that sustains aquatic life.