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Slabstock Foams

Slabstock foams, like most foams, are prepared from polyols, isocyanates, and water. Along with these basic ingredients are added catalysts, surfactants, coblowing agents, flame retardants, antioxidants, and sometimes colorants. All of the nonisocyanate components are premixed in combinations that are convenient for the foam manufacturer and then vigorously mixed with the isocyanates. The mixed and reacting system is applied to a moving belt (Fig. 6.1). The large foam slab (sometimes also referred to as a "bun") is subsequently cut into manageable sizes, set aside in a well-ventilated location to cool and complete its cure. The actual geometry of a particular slab foam production line may vary from that diagrammed in Figure 6.1 but will in essential details be similar. The ventilation requirements are due to the substantial exotherm from heat of reaction and the volatility of the polyurethane components, particularly isocyanates, catalytic amines, and small-molecule impurities from the polyol production—mostly aldehydes. Control of the exotherm is

Slab foam process. The image does not show the feed tanks, ventilation, or substantial amount of electronic controls involved. Reprinted with permission from Ref. [3]. © John Wiley & Sons, Inc.

FIGURE 6.1 Slab foam process. The image does not show the feed tanks, ventilation, or substantial amount of electronic controls involved. Reprinted with permission from Ref. [3]. © John Wiley & Sons, Inc.

critical to prevent oxidative discoloration of the foam center referred to as "scorch." In the extreme, a foam may even catch fire if the foam interior temperatures are not controlled.

Typical foam formulations that could be attempted by an experimentalist with a high probability of success would be exemplified by those found in Table 6.1. These two recipes are distinguished by the amount of water and the amount of isocyanate, in this case toluene diisocyanate (TDI). The effect of increased water and concomitant TDI is to (i) reduce the foam density with higher water (increase the amount of bubble formation due to the isocyanate-water reaction that produces C02 (Chapter 3), (ii) increase the foam stiffness due to additional urea formation that further reinforces the foam with a high modulus block (Chapter 4), and (iii) increase the exotherm produced by the foam as it rises.

TABLE 6.1 Representative slab foam recipes. To make these foams in a lab, place all components in a well-ventilated fume hood with an accurate scale. Mix all but the isocyanates in a beaker. Add isocyanate to beaker, mix vigorously for 10 S, and pour into a box adequate for the volume expansion. Clean removal of the foam from the box will probably require the use of a release paper liner. Perform in a well-operating chemical hood with appropriate personal protections

Representative slab foam recipes. To make these foams in a lab, place all components in a well-ventilated fume hood with an accurate scale. Mix all but the isocyanates in a beaker. Add isocyanate to beaker, mix vigorously for 10 S, and pour into a box adequate for the volume expansion. Clean removal of the foam from the box will probably require the use of a release paper liner. Perform in a well-operating chemical hood with appropriate personal protections

The stoichiometry of the reactive components is matched so that the total equivalents of isocyanate and isocyanate-reactive components are matched. There is a 10% excess equivalence of TDI added in this case (110% of equivalence referred to as "index"). This is done to assure that all of the active hydrogen components are tied up to make a complete network. Some of the excess isocyanate is volatilized as previously mentioned. Some of the excess is employed to cover any loss of isocyanate functionality due to dimer formation, unintentional urea formation due to reaction with adventitious water, and cross-linking isocyanate-isocyanate reactions due to excess concentration fluctuations (Chapter 3). Formulations of Table 6.1 can be scaled up by proportionality, but the experimentalist should always be cautious due to the increasing adiabatic heating of the foam and its high insulation properties.

The quantification of hard segment in the foams of Table 6.1 can be described by either the percent hard segment or the hard segment length. The percent hard segment is simply the sum of grams of water and isocyanate divided by the total mass of the foam. The length of the hard segment can be calculated from the sum of molecular lengths making up the hard segment times the average hard segment oligomer number. Alternatively, it can be easier to simply refer to the hard segment length based on the average number of urea repeat units between polyol soft segments easily approximated by the equivalents of water divided by the equivalents of polyol in the formulation [4]. Table 6.2 provides an illustration of this variation for the foam recipes of Table 6.1. It is clear that there are no fractional hard segment lengths such as 2.2 or 4.4. This number merely indicates that most hard segment lengths will be about 2 or 4. As discussed in Chapter 5, one can expect that there will be a significant amount of isocyanate that is reacted with only polyol and there will be hard segment blocks with a significantly larger number of repeat units greater than 2 or 4, but these will average out to approximately 2.2 or 4.4.

The properties of the two foams defined in Tables 6.1 and 6.2 will be primarily determined in these particular cases by their density and the percent hard segment. On reflection, it is apparent that these two quantities are not independent of each

TABLE 6.2 Alternative methods of describing the hard segment content of a polyurethane foam

Alternative methods of describing the hard segment content of a polyurethane foam

TABLE 6.3 Property range of a conventional slabstock foam made from the recipes of Table 6.1

Property range of a conventional slabstock foam made from the recipes of Table 6.1

other since more water increases percentage hard segment and reduces foam density as more C02 and heat are generated. The increasing temperature and gas pressure creates expansion pressure on the growing foam and so reduces density. However, while additional water creates additional hard segment and stiffening of the foam, a higher foam density obtained by reducing water will also make for harder foam by virtue of more massive struts making up the foam structure [5]. A range of selected properties that might be obtained from foam recipes 1 and 2 are given in Table 6.3.

High-resilience ("HR") slabstock foams are made by a similar process to the conventional slabstock foams described earlier, but there are substantial formulation and performance differences. The HR designation reflects the increased elasticity of these foams as measured by a ball rebound test (Section 5.3.1, ASTM test D2632 [6] replacing D3770). A standardized steel ball is dropped from a prescribed height and the rebounded height measured. A typical conventional foam may have a rebound resilience of 40-50%, while HR foams will usually be greater than 55%. HR foams are also referred to as "comfort" foams due to this enhanced energy return upon compression. The uses of HR foams are typically as higher-quality mattresses and furniture cushions. From the formulation standpoint, the biggest difference is the polyol structure. As shown in Table 6.1, conventional slabstock foams typically have hydroxyl equivalent weight on the order of 1000 g/eq and are triols made entirely from propylene oxide (PO). All PO polyols have only secondary hydroxyls to react with isocyanates. In contrast, HR slabstock foam employs polyols with functionality of three or greater and hydroxyl equivalent weight of between 1500 and 2000 g/eq. Since higher equivalent weight polyols have less functionality to react with isocyanate per unit mass, they are intrinsically less reactive than lower equivalent weight polyols of the same structure. To achieve similar reaction kinetics to conventional slabstock polyols, PO polyols are tipped with ethylene oxide (EO) to make triblock EO-PO-EO polyols having a significant amount of primary hydroxyl functionality. Primary hydroxyls are about a factor of 3 more reactive than secondary hydroxyls and are part of the design options for optimizing foam rise and cure to meet a manufacturer's specifications. Despite the presence of primary hydroxyls, HR foam may still collapse if there is insufficient molecular weight build to prevent foam cell coalescence. In this case, a cross-linker like diethanolamine (DEOA—Fig. 6.2) may be incorporated into the formulation [7]. DEOA has a secondary amine and two primary hydroxyls creating a low-molecular-weight cross-linking agent for isocyanates and a means of rapidly building molecular weight. DEOA not only builds foam stability as a cross-linking agent, but it can also have a significant negative effect on the ability of foam cells to rupture and reticulate throughout the foam body. The effect will be to reduce the foam's airflow and increase foam stiffness due to hydraulic resistance to compression. This source of foam load bearing is not usually desirable since it is an artifact of processing and will change with use and age. For this reason, while DEOA is a very common constituent of foam formulations, it is used in small amounts, usually comprising less than 1% of the total isocyanate-reactive composition. Similarly, a silicone surfactant that does less to stabilize the cell wall will usually be added (medium to low efficiency) to facilitate cell window drainage to the cell interstitial boundaries (see Section 6.2.1.2).

While hydraulic resistance to foam compression is usually avoided, formulators often utilize copolymer polyols to increases foam stiffness and load bearing in HR foam (Section 2.1.4). A representative HR foam formulation and properties are provided in Table 6.4.

The structure of the common polyurethane foam cross-linking agent dietha¬nolamine (DEOA).

FIGURE 6.2 The structure of the common polyurethane foam cross-linking agent diethanolamine (DEOA).

TABLE 6.4 HR foam formulation and representative properties

HR foam formulation and representative properties

 
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