Understanding
FOAM properties
Reprinted by permission | UDM UPHOLSTRY DESIGN & MANUFACTURING | DECEMBER 1993

Being knowledgeable about the different physical properties of foam can help you make a more educated buying decision.

Polyurethane foams are manufactured in a complex process in which virutally hundreds of different chemical reactions take place simultaneously. Several non-chemical, physical reactions also occur.

Although the manufacture of polyurethane foam requires complex chemistry, it isn't necessary for you to understand all the details of that chemistry to produce furniture. One point that's readily understandable is that advanced foam technology isn't always unpredictable.

As foam manufacturing has evolved, it has grown in its ability to control the major physical properties of furniture foams. These physical characteristics potentially hold far more interest for furniture manufacturers than chemical reactions.

Lession #1

The first physical property you should be concerned with is density. Traditionally, polyurethane foam density has been measured in pounds per cubic foot, abbreviated PCF. One good way to get a frame of reference for density is to think in terms of cubes of foam measuring exactly 12 inches on each side. This 12-inch cube is exactly one cubic foot in volume.

If this 1.0 cubic foot weighs 1.0 pound, its density is said to be 1 pound per cubic foot, or 1.0 PCF. If this cube weighs 2.0 pounds, it's said to have a density of 2 pounds per cubic foot or 2.0 PCF.

Density is directly related to foam cost. When you buy foam, you're buying chemicals. The higher the density you specify, the more chemicals you'll get, and the more it will cost.

Density also is related to the durability of field performance of foams. As the density of unfilled flexible polyurethane foam decreases so does its durability.

As a rule of thumb, the higher the density, the higher the cost, and the better the performance.

In the past, although lower density foams were less expensive, they produced complaints in the field because they were extremely light in weight. To solve the weight problem, foam technicians added inert fillers to foam formulations. These inert fillers raised the density and the weight of the cushions, but performance of the filled foams left much to be desired.

Subsequent research has found the durability of filled foams varies according to the density of the foam before the filler is added. In general, an unfilled 2.0 PCF foam is more durable than an unfilled 1.2 PCF foam.

Filling a 1.2 PCF foam to a density of 2.0 PCF generally will not make the 2.0 PCF-filled foam any more durable than the 1.2 PCF foam. To some degree, fillers detract from the physical properties of foam. Because of what fillers do to foam formulations, the 2.0 PCF filled foam in the example is likely to be less durable than its 1.2 PCF counterpart.

The hardness or softness of the foam is probably the second most important foam property. This is measured by a property defined as the IFD of the foam. IFD stands for indentation force deflection. By definition it's the amount of load, in pounds, necessary to press a 50-square-inch indenter foot into the foam sample to a percentage of the foam thickness.

The most used IFD value is the 25 percent IFD. On a 4-inch-thick piece of foam, the 25 percent IFD would be the amount of force required to indent the 50-square-inch deflector foot exactly 1 inch into the piece. Any IFD value must be accomplished by a percentage deflection value, a sample thickness value, and a sample length and width because all of these values affect the outcome.

Another commonly used physical measurement is called modulus or support factor. In addition to the 25 percent IFD measurement, a second IFD measurement is taken with the foam compressed to 65 percent of its original height. The ratio of the two IFD measurements - the 65 percent IFD divided by the 25 percent value - is the foam's modulus.

Measuring Support

A polyurethane foam with a 25 percent IFD of 28 lbs./50 square inches and a 65 percent IFD of 56 lbs./50 square inches would have a modulus of 2.0.

This measurement gives a reading of the relationship between the foam's ability to support weight and prevent bottoming out. The bigger the difference between the 65 percent IFD and the 25 percent IFD, the higher the modulus, and the better the foam is at providing support.

There is a school of thought contending that foams with a modulus less than 1.85 are undesirable. This can't be scientifically supported, however it's known that as modulus values approach 1.0, foams become "mushier." At this level the materials of foam cusion is resting on, the springs for instance, can be felt.

Try this test

The next most important foam property is fatigue softening. Fatigue softening is one of the measurements of durability and is extremely important. To understand how quickly foams fatigue-soften, take a cushion and hand flex a corner as hard and as fast as you can for about 1-1/2 minutes.

Then wait for one minute and feel the foam on another, unflexed corner. Compare the feel of the unflexed corner to the flexed corner. The flexed corner will feel softer. Density is one of the major controllers of fatigue softening.

Compression set is the tendency of foams to remain compressed after the force of compression is removed. This is another important property of furniture foams. As the compression set value increases, regardless of the method of measuring that value, the tendency for corresponding foams to show problems with compression set increases.

The magnitude of a "good" compression set value and how it is measured is open to discussion. But manufacturers should always be at least suspicious of compression set values above 15 percent.

A basic understanding of these foam characteristics is vital for effective upholstry manufacture. To get the maximum benefit from the application of any these values, manufacturers should work closely with their foam suppliers to develop limits and ranges that both parties find acceptable.

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