DENSITY IN FLEXIBLE POLYURETHANE FOAM (FPF)
LESSON PLAN FOR 50 MINUTE CLASS
1. The student will be able to differentiate between flexible and rigid polyurethane foam in composition and uses.
2. The student will be familiar with the terms associated with the use of flexible polyurethane foam.
3. The student will be able to calculate the density of a polyurethane foam sample.
1. InTOUCH; Volume 1, Number 1; February 1991
2. InTOUCH; Volume 1, Number 2; May 1991
3. Flexible Polyurethane Foam Glossary
I. (3-5 minutes) Introduce the subject of polyurethane foam.
Foams are essentially huge networks of tiny three-dimensional cells resembling a honeycomb structure shaped in a pentagonal dodecahedron configuration (twelve five-sided planes).
Polyurethane foams are divided into two families: flexible foam and rigid foam. Flexible foams are primarily used in cushioning applications. As you walk, drive a car, sit, or sleep, flexible polyurethane foams (FPFs) are likely providing comfort. Rigid polyurethane foams are primarily used as thermal insulation in refrigerators, building panels other similar insulating applications. It is sometimes also used as a wood replacement where visual details can be created in a mold rather than carving.
This curricula addresses flexible polyurethane foam (FPF) characteristics and how they relate to cushioning applications.
In FPF, the cells are made up of two structural parts: cell walls (called struts) and open window areas (called voids). This strut and void structure allows air to pass through the foam when force is applied. The elasticity of the struts acts as a shock absorber and allows the foam to recover shape after compression in the same manner as shock absorbers perform in a car.
On the other hand, rigid foam cells have much the same structure, but windows in the cell walls are closed, restricting air flow and improving thermal efficiency.
II. LECTURE (20-25 minutes)
A. Discuss the difference between flexible and rigid polyurethane foam. As you are discussing this, pass around samples of each for the students to examine. Flexible polyurethane foam (FPF) is mainly used for furniture cushioning, carpet underlay, auto seating, and bedding. Rigid polyurethane foam is mainly for insulation. It is used in refrigerator walls, as well as building insulation panels. To a lesser degree, rigid foam can also be found in decorative applications such as molded picture frames, interior trim, exterior building shutters and other uses where visual details can be created by molding rather than intricate carving.
B. KEY INGREDIENTS TO ALL FLEXIBLE FOAM APPLICATIONS
1. Support -- The foam has to be able to support the proper amount of weight to properly cushion an object or person. Thus, in upholstery, good support from foam means that cushions don't "bottom out" or compress to the point where they no longer hold up the weight of the person.
2. Comfort -- Foam cushioning has to feel good to the user and provide not just cushioning but also comfortable use. Comfort can be defined as the ability of the cushioning structure to deflect at the surface and to conform to body shape, preventing a concentration of pressure on the body without bottoming out.
3. Durability -- The foam has to hold up through use without losing its original properties. Durability, as applied to flexible foams, refers to how well a foam retains its load bearing capacity and shape with use. Most measures of durability are done with laboratory-scale tests.
C. FOAM PRODUCTION
1. Slabstock form process (Put transparency [T1] on overhead projector)
In this process, the chemical mix is poured onto a moving conveyor, where it is allowed to react and expand. The foam rises to form a "slab' anywhere from two to four feet high. The continuous slab is then cut into "buns", stored, and allowed to cure for up to 24 hours. The cured foam is subsequently fabricated into useful shapes. Most foams for use in furniture, bedding and carpet cushion are produced this way.
2. Molded foam process (Put transparency [T2] on overhead projector)
This is a process where individual items are produced by pouring foam chemicals into specially shaped molds and allowing the foam reaction to take place. This process is used primarily for automotive cushioning, although much contract furniture and a few residential upholstered designs utilize molded cushions.
3. Environmental Impact -- During the foam fabrication process, chemicals react exothermically giving off considerable heat. In order to control reaction temperatures, auxiliary blowing agents such as freon were once used. The polyurethane foam industry in the United States has been successful in finding chemical and mechanical process substitutes for freon resulting in the virtual elimination of CFCs in flexible foam processing. The Clean Air Act Amendments of 1990 have further pushed the industry to reduce all airborne chemical emissions.
4. OTHER ADDITIONS TO THE BASIC FOAM CHEMICAL MIX
a. Flame retardant additives -- Used to improve a foam's resistance to ignition or burning. (Unfortunately, tend to have a negative influence on the comfort, support, and durability of the foam.)
b. Fillers -- Increase the weight of the foam and often add to support, but can possibly have a negative influence on the durability properties of the foam.
c. Colorants -- Used for visual identification purposes, allowing foam producers and their customers to quickly recognize foam grades. Colorants do not affect physical performance.
d. Antistatic Additives -- Reduces static electricity build-up for use in critical applications where static discharge could have a detrimental effect (packaging for computer chips, carpet cushioning in computer rooms and hospitals).
e. Antibicrobials -- Broad spectrum antimicrobials effective against a variety of bacteria, fungi and yeast. For application where FPF is to be used in moist, warm environments conducive to growth of mold and mildew.
D. DENSITY OF FLEXIBLE POLYURETHANE FOAM
1. WHAT IS DENSITY? -- (Put transparency [T3] on overhead)
Density is a measurement of the mass per unit volume. Measured and expressed in pounds per cubic foot (pcf) or kilograms per cubic meter (kg/m3), it is one of the most important of all form properties. Remember that flexible polyurethane foam is made up of struts and voids. Since voids are empty spaces, when you weigh a sample of foam, you are actually weighing the struts, not the voids. The diameter and length of the struts represent the mass. The physical shape of the struts and the absence or presence of windows (voids) also play a major role in foam physical performance. 2. HOW DENSITY AFFECTS FOAM PERFORMANCE
a. Foam surface firmness is independent of foam density. High density foams can be produced very soft and low density foams can be made very firm. Therefore, there is no such thing as a "hard" or "soft" density. High density foam products generally offer a great deal of support, but they may actually be fairly soft foams.
b. There is a difference between firmness and support. Firmness (25% IFD) is a measurement of surface feel of the foam. Support is the foam's ability to "push back" against weight and prevent the foam from "bottoming out." Higher density foam helps prevent the feeling of having the foam collapse beneath the body weight in an end use application.
c. The support factor, foam durability, flex fatigue and compression set are other terms related to density and how it affects foam performance. You will learn about them in your homework assignment.
IV. (15-20 minutes) Divide the class up into small groups. Give each group samples of flexible polyurethane foam and a list of flexible polyurethane foam applications (located after this lesson plan). Ask each group to examine the samples and then compare the density of each sample. Then, have them decide which application from the list this sample would be best suited for. Remind them to keep in mind when high density would be appropriate and when low density would be best.
V. (5-7 minutes) Place the "Typical Density Ranges for Different Foam Applications" transparency [T4] on the overhead projector and have the groups compare their decisions to the table. Discuss the table and then give out assignments for next class period listed below.
1. Samples of flexible and rigid polyurethane foam to use as examples
2. Glossary Terms handout
3. Flexible Polyurethane Foam Applications handout (Both of these are located after this lesson plan)
4. Overhead transparencies
5. Overhead projector
1. Learn the terms related to flexible polyurethane foam listed on the handout. Be prepared to be quizzed on these.
2. (Optional) Learn more about flexible polyurethane foam by checking the Polyurethane Foam Association's World Wide Web site at http://www.pfa.org
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