PIPA polyols are related to PHD polyols except that polyurethane rather than polyurea filler phase is produced in a polyol. The process is relatively simple, and several foam producers are known to produce it for captive use rather than purchase alternative-filled polyols. The chemistry is relatively straightforward, though optimization requires significant control. The particle is produced through the reaction of a polyisocyanate such as TDI with a short-chain polyol such as TEA as illustrated in Fig. 2.38. The polyol, the TEA, and the catalyst are premixed,
FIGURE 2.38 Preparation of a PIPA polyol.
and the TDI is added rapidly with vigorous mixing . As the particle forms in the polyol medium, it can exceed its solubility limit and precipitate. Surface isocyanate groups allow for continued growth of the particle from reaction with TDI and TEA though this occurs at a slower rate. In addition, at all times, free isocyanate groups can react with hydroxyl functionality from the polyol. In fact, if the isocyanate reaction with the polyol is not closely controlled, undesirable properties are obtained. For instance, if the polyol reaction is encumbered by the presence of too many polyol secondary hydroxyl end groups, the particles will not stabilize in the polyol medium and will develop a size distribution with large average size. If there is too much reaction with the polyol, the size distribution will be small, but the filled polyol will have a very high viscosity.
In practice, this problem is controlled via a technique commonly employed in latex emulsion polymerization, which is to seed the medium prior to isocyanate addition. Thus, to the base polyol-TEA-catalyst, an addition is made of small seed particles in the base polyol. When the TDI is added, particle formation begins preferentially on the suspended seeds, resulting in a high density of preformed uniform nucleation sites from which the PIPA particles can grow. The result is a significantly more uniform particle distribution centered at smaller sizes. The reaction of base polyol on the seed isocyanate surface also serves to limit the further growth of the particles. Controlling the concentration of seed particles and the size and distribution of seed sizes in the initial state helps obtain an optimum final result.