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Surfactancy

Virtually all polyurethane foams are made with the essential addition of surfactants. The role of the surfactant is still not well understood in specific detail, but a sufficient amount of controlled structure-property experimentation has been done to provide

(a) Example reaction for making a catalytic polyol. The reaction is usually base catalyzed, (b) Energy minimized space filling structure for a reactive catalytic polyol. The arrow indicates the position of the central catalytic tertiary amines. The alkoxylated aminines are usually much less catalytic due primarily to steric hindrance as well potentially electrostatic factors associated with local ether associations. (See insert for color representation of the figure 6.8(b).)

FIGURE 6.8 (a) Example reaction for making a catalytic polyol. The reaction is usually base catalyzed, (b) Energy minimized space filling structure for a reactive catalytic polyol. The arrow indicates the position of the central catalytic tertiary amines. The alkoxylated aminines are usually much less catalytic due primarily to steric hindrance as well potentially electrostatic factors associated with local ether associations. (See insert for color representation of the figure 6.8(b).)

TABLE 6.10 The effects of catalyst strength on the polyurethane foam process and foam

The effects of catalyst strength on the polyurethane foam process and foam

good understanding of desirable surfactant features for a specific foam [23]. The surfactant requirements for closed-cell rigid foam are different in detail from those of an open-cell flexible foam, but the general surfactant actions are the same.

Conventionally, it is understood that the surfactant plays no intermediary role in the reactions of isocyanates with the other foam components [24]. This is inferred by the lack of a kinetic effect in the specific reaction rate as a function of surfactant composition. However, the surfactant plays a crucial role on the foam process kinetics and eventual structure and properties. The surfactant is believed to (i) lower surface tension at the polyurethane-air interface promoting the generation of bubbles that evolve into foam cells, (ii) emulsify the incompatible polyurethane components (which does affect polymerization rate by influencing available concentration), and, most importantly, (iii) control the stability of the cell polymer surfaces. The surfaces must specialize into cell struts and windows upon packing. At a specific point of the foam rise, the cell windows must drain into the cell struts and open such that airflow through the foam is at the desired level. The details of this process are strongly dependent on the details of the surfactant structure.

The vast majority of surfactants used for making flexible polyurethane foams are of the class termed broadly as silicone surfactants. Silicone surfactants consist of polydimethylsiloxane (PDMS) backbones grafted with polyethylene oxide-co-polypropylene oxide side chains (Fig. 6.9) [25, 26].

In the early stages of foaming, the isocyanate, polyols, and water may all be mutually immiscible. The surfactant can promote initiation of the foaming reaction by specifically lowering the surface tension between the isocyanate and water phases. The structure of the silicone certainly affects this as the hydrophobic PDMS and the hydrophilic EO balance can lower the surface tension and increase the probability of reaction. As the reaction proceeds, the system advances to a single phase, and the surfactant's role as an emulsifying agent ends.

The physical mixing that occurs introduces bubbles into the reacting polyurethane components. In the absence of a surfactant or in the presence of a poorly chosen surfactant, these bubbles will coalesce, rapidly exceed the bubble wall's tensile strength,

General structure of a silicone surfactant as might be used for polyurethane foaming.

FIGURE 6.9 General structure of a silicone surfactant as might be used for polyurethane foaming.

Suggested polyhedral structures found in polyurethane foams based on space filling considerations—left, pentagonal dodecahedron and, right, tetrakaidecahedra— compared to actual foam structures (Fig. 6.5).

FIGURE 6.10 Suggested polyhedral structures found in polyurethane foams based on space filling considerations—left, pentagonal dodecahedron and, right, tetrakaidecahedra— compared to actual foam structures (Fig. 6.5).

and escape from the foam. If the foaming mixture has advanced its molecular weight, the reacting mixture may initially rise but will collapse. An efficient surfactant will serve to stabilize the foam structure throughout the foam process and will also control film dynamics of the growing bubbles within the foam that lead to open and reticulated cells.

In the presence of adequate surfactancy, catalysis, and a suitable mixture of polyurethane components, the foam will begin to rise. As the bubbles grow, the molecular weight of the polymer will increase providing sufficient polymer tensile properties to contain the growing internal gas pressure. When the volume fraction of bubbles exceeds 74%, the bubble growth will become limited by bubble-bubble physical interaction. The bubble will cease to be spherical and will assume a polyhedral shape as can be made out from the foam shapes of Figure 6.5. There is some controversy about the nature of the polyhedra formed (pentagonal dodecahedra or tetrakaidecahedra based on space filling surface minimization; Fig. 6.10); however, it is clear upon inspection that real foams are not regular shapes.

TABLE 6.11 Effects of silicone surfactant variables on foam process and properties

Structural feature

Effect on foaming

Polyether content

Increase in surface tension

Siloxane content

Increase—decreases surface tension

Smaller bubble size

Ratio of siloxane to polyether

Increase—lowers surface tension

Increase—increases bubble count

Too high can be antifoaming, causing foam

collapse

Too low can result in too low film elasticity,

causing foam collapse

Increase of molecular weight at constant

No effect on surface tension

siloxane/polyether

No effect on bubble count

No effect on bubble size

Higher film elasticity/slower drainage/more

closed cells

The successful fabrication of polyurethane foam requires the artful balance of polymerization kinetics, competing reactions, and physical processes racing to a finished product. Small maladjustments in achieving this balance are the difference in a beautiful finished foam product and a solidified puddle of raw material. The surfactant structure-property relationships for making successful foams have been broadly defined in the literature based primarily on model compound studies. This has necessarily been the case since the vast majority of silicone surfactants are produced by manufacturers who protect structures as intellectual assets. Manufacturers instead define the surfactant in terms of efficiency in certain foam classes (i.e., slabstock, hot molded, etc.) and in terms of effectiveness at emulsification, cell size regulation, usefulness in polyester foams, copolymer polyols, specific isocyanates, and so on. Table 6.11 provides general structure-property relationships for silicone surfactants as applied to flexible foam fabrication.

 
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