Polyurethane Foam

A couple of years ago, as I was cleaning my bedroom, I came across a wad of shriveled foam in a small paper cup. It immediately brought back memories of one of my high school science experiments. I remembered pouring two liquids into the cup and watching the mixture bubble and rise like bread in an oven. What I had created was polyurethane foam. Being the pack rat that I am, I couldn't bring myself to toss my old science project out.

As I learned later, there are many types of polymer foam. "People think a foam is a foam is a foam," says Robert Luedeka, executive director of the Polyurethane Foam Association. "And that's not the case."

For example, the foam cups that hold your coffee and other hot beverages are typically made of polystyrene. So are the foam peanuts used in packaging. And the pool noodles kids splash around with during the summer? That's polyethylene.

Polyurethane foam, in contrast, is the soft stuff that fills many sofas and mattresses. It is also the hard stuff that is used to insulate buildings and repair cracks around the home. These two types of foam are known as flexible foams and rigid foams, respectively, and they account for most of the polyurethane foam market.

There are two other types of polyurethane foam with more specialized uses: microcellular foam, which may be used to make car steering wheels or line the insides of athletic helmets, and elastomeric foam, which is typically used to make the outer sole of many types of footwear, including athletic shoes.

These four classes of polyurethane foam, all with extremely different physical properties, are created by varying a basic addition polymerization reaction involving a diol or polyol, a diisocyanate, and water.

The diisocyanate reacts with the diol or polyol to create the urethane polymer. Water reacts with some of the isocyanate groups to produce carbon dioxide gas, and the bubbles get trapped in the viscous liquid as it polymerizes, expands, and solidifies.

On a large scale, catalysts and surfactants can be added to boost the reactions and control the foaming process. A wide range of other additives, including stabilizers, dyes, fire retardants, and fungicides, may be added to meet specific performance demands.

Auxiliary blowing agents, such as hydrofluorocarbons, liquid carbon dioxide, and acetone, are also typically used to prepare the foam. Ultimately, the kinds and amounts of raw materials used in the manufacturing process help determine how the final product performs.

Flexible foams have mainly open cells, formed by gas bubbles that have popped. Air can pass through the foam easily, resulting in a soft, resilient, flexible material. In rigid foams, most of the cells stay closed. The material is thus harder and less resilient. Controlling the proportion of open cells to closed cells during the production process is one of the ways that the properties of foam can be manipulated, adding to the material's versatility.

Polyurethane foam products are everywhere. "You live with it 24 hours a day," Luedeka says. "It's underneath the carpet you walk on; it's in the car when you drive home. When you sit in your living room, you're on it; you sleep on it at night. It's a pervasive material."

Polyurethane foam also figures in some highly specialized applications. For example, the rigid type is used to insulate the external fuel tank on space shuttles. Following the Columbia disaster in 2003, engineers from the National Aeronautics & Space Administration determined that a piece of foam broke off the external fuel tank and struck the leading edge of the spacecraft's wing (C&EN, Oct. 31, 2005, page 26).

The flexible form of polyurethane foam is used in contraceptive sponges. It's also found in aquarium filters, on the front of stereo speakers, and in diaper waist bands.

Research continues to be carried out to improve the material. One advance in flexible polyurethane foam technology was the introduction of viscoelastic, or memory, foams. After compression, these foams return to their original shape more slowly than conventional foams. Memory foams are largely used in pillows and mattresses for added comfort.

Researchers also are trying to replace polybrominated diphenyl ether, a flame retardant that is added to polyurethane foam and that has been found in increased levels in human blood and breast milk over the past 25 years.

One environmental achievement is that most flexible polyurethane foam scraps are now recycled into carpet padding, reducing the burden on landfills.

And speaking of landfills: That day I reconnected with my high school science experiment, I put my homemade foam aside and continued cleaning my bedroom, throwing out many other mementos. Eventually, I came back to the foam. It had brought back wonderful memories, but alas, it too had to go.