Mattresses and Bedding
As shown in Figure 7.1, mattresses and bedding (pillows, bolsters, mattress toppers) are a large and important industry segment for flexible foams. The market for mattresses is sensitive to the macroeconomic environment, and in some years, mattresses and bedding will in fact be the largest industry segment for slabstock foams . The number of mattresses (and so volume of flexible foam) and the type of mattresses (better or poorer quality/more or less foam) will vary significantly from year to year. Mattresses and bedding are almost entirely based on slabstock polyurethane foam production. The types of slabstock foams used, however, do cover a distribution of specifications and thus formulations.
In Figure 7.5, the category of viscoelastic foam (also called "memory foam") is introduced, and this class will be fully developed in the following. The quality of the foam is in part a function of foam density. Many consumers will make an informed decision to purchase mattresses with higher-quality foam having experienced the permanent deformations of mattresses based on low-density/low-quality foam (Fig. 7.6).
Mattresses are engineered structures [22, 23]. They are typically an ordered array of metal springs (the inner spring) covered by several inches of polyurethane foam. Depending on the size and density of foam covering the inner spring, a mattress may contain 10 lbs of polyurethane foam. Some mattresses (both high and low quality) will employ a polyurethane core in replacement of the inner spring. A high-quality foam core may utilize foam density as high as 4 lbs/ft3.
Over the past decade, a significant segment for flexible foam growth has been viscoelastic foams [24,25]. Unlike conventional or high-resilience foams that are valued for their high efficiency in immediately returning energy, viscoelastic foams have a delayed recovery to deformation and are also called "low-resilience foams." They are also called "memory" foams because of their ability to retain the shape of what was just compressing them (Fig. 7.7). Given time, a viscoelastic foam will recover its
FIGURE 7.5 Approximate breakdown for the types of foams used in the manufacture of mattresses. Al foams represent types of flexible slabstock foam manufacture.
FIGURE 7.6 Approximate breakdown of the flexible foam market based on foam density. This data can fluctuate based on economic conditions with cheaper/lower density foams increasing in poorer economic conditions and more expensive/higher density foams increasing in better economic conditions.
FIGURE 7.7 Illustration of the delayed recovery response of a polyurethane viscoelastic foam. Image courtesy of The Dow Chemical Company.
original shape. The behavior of viscoelastic foam is a result of the underlying polymer structure and is controlled by the formulation that the foam designer utilizes. Specifically, when designing a viscoelastic polyurethane foam, the designer will formulate to result in a broad range of molecular weights between cross-links [26-29]. The effect of this molecular weight distribution is to broaden the glass transition temperature such that the molecular motions of the polymer chains have a broad range of relaxation times. In Chapter 4, the effect of block copolymer structure on the glass transition temperature was discussed, but the effect of cross-link density on the glass transition temperature of a rubber is roughly given by Equation 7.1 where Mc is the molecular weight between cross-links and k can be treated as a constant :
Thus, as the molecular weight between cross-links evolves to a broad distribution, the glass transition temperature will likewise reflect the breadth of that distribution. This will be reflected phenomenologically as a slow recovery of shape after deformation, particularly when the T is peaked between 0 and 15 °C (Fig. 7.8). Rheologically, the glass transition temperature reflects the physically encumbered chain motions being activated over a broad range of temperatures. From a foaming point of view, the broad and elevated tan delta slows the foam rise and cell reticulation ("blowoff') since the low molecular weight between cross-links will enhance gelation over blow reactions. The result can be reduced airflow of viscoelastic foams due to a greater number of intact cell windows (Fig. 7.9). The reduced airflow can contribute to the slow recovery response of the viscoelastic foam, but its contribution usually evolves as the foam ages. From the manufacturing point of view, the production of viscoelastic foam is not different from the production of other slab foams except that the
FIGURE 7.8 Shear modulus and tan delta spectrum of (a) a viscoelastic and (b) a comparative high resilience flexible foam. The position, width and amplitude of the tan delta peak are typical of those whose applications demand viscoelastic response.
designer will produce the desired effect by blending two polyols with substantially different equivalent weights. For instance, in the representative formulation of Table 7.2, one polyol has a hydroxyl equivalent weight of about 140 g/eq, while the other is approximately 1000 g/eq. Miscibility of the polyols assures that there will be a broad distribution of molecular weights between cross-links. While viscoelastic foams have by far made their largest impact in mattresses and bedding, they are beginning to appear in some automotive seating systems as foam designers have learned to broaden out the tan delta envelope to the required temperature range specified by manufacturers .
FIGURE 7.9 Scanning electron microscope image of a typical viscoelastic polyurethane foam.
TABLE 7.2 Representative formulations and property ranges for viscoelastic foams prepared at three different densities