The polymerization of styrene and acrylonitrile to form SAN in polyol was first documented by Stamberger in the 1960s . The polymerization is easily accomplished using AMBN 2,2'-azodi(2-methylbutyronitrile) or AIBN as an initiator for the radical polymerization (Fig. 2.34).
The limited solubility of SAN in polyol causes it to phase separate into domains capable of scattering light. This results in filled polyol being milky white. The phase-separated polyol phase is referred to as the serum since it still contains SAN polymer and oligomers up to the solubility limit. During the polymerization, the radical initiators may abstract hydrogen from the polyol, particularly the hydrogen on the tertiary carbon of polypropylene oxide, and provide a radical site on the polyol chain (Fig. 2.35). These carbon-centered radicals provide new initiation sites
FIGURE 2.34 Polymerization of SAN using azobisisobutyromtnle (AIBN) as an initiator.
FIGURE 2.35 Generation of a carbon centered radical on a propylene oxide polyol. The radical center can serve as a polymerization initiator or chain terminator.
FIGURE 2.36 Examples of macromer structures.
for radical polymerization of SAN polymer and create a graft stabilization mechanism keeping the SAN dispersed in the polyol . The polymer in the serum causes the viscosity of the filled polyol to increase such that it has a useable limit of 20-25% solids before it becomes unusable to foam manufacturers due to poor flow and mixing with other foam components having significantly lower viscosity.
Continuing innovation has focused on achieving ever smaller particles, low color formation, higher solids at lower viscosity, and improved VOC profile. Polyol viscosity, directly related to solid content, has been an area of particular innovation and patent activity. Breakthrough performance has been established by the development of stabilizers or "macromers" (macromolecular monomers) . These molecules typically are A-B functional monomers with vinyl and hydroxyl functionality. While reacting readily into the SAN polymer structure, they can provide a new stabilization mechanism, preventing agglomeration or settling, without requiring the formation of radical sites on the polyol chain. Typical macromer structures are presented in Figure 2.36. The hydroxyl functionalization is related structurally to the host polyol, while the vinyl component is polymerized with a short SAN chain. Artful design of
FIGURE 2.37 Scanning electron microscope image of particles formed in co-polymer polyol synthesis. The particle size uniformity points to the exquisite control and optimization of the process. Image by Robert Vastenhout; The Dow Chemical Co.
macromer structures, polyol design, and processing have allowed solid contents greater than 50% solids, with viscosities of ca. 5000 mPas, and weight average particle sizes under 1 urn (Fig. 2.37). The copolymer polyols have adjusted hydroxyl equivalent weight of between 2500 and 3000 g/OH eq. The polyols usually used for dilution in foam formulation are high-functionality polyols with equivalent weight between 1500 and 2000 g/eq. The polyols are usually EO-capped PO polyols with 15-20% EO content to assure rapid polyurethane formation in molded foam operations.