The low cost and ubiquity of polyols has generated innovation in features and additives to add value to downstream polyurethane processing. One innovation has involved using the polyol as a polymerization medium and then using those polyol-copolymer mixtures to provide a downstream value. Three so-called copolymer polyols have been developed and found significant commercial value. These are (i) styrene-acrylonitrile (SAN)-filled polyols , referred generically as copolymer polyols, prepared via radical polymerization of vinyl monomers; (ii) step growth copolymer polymerizations of isocyanates and amines like hydrazine called poly/zarnstoff dispersion (PHD) polyols ; and (iii) polyfsocyanate polyaddition (PIPA) polyols made by reaction of an isocyanate and an alkanolamine like triethanolamine (TEA) . PHD polyols result in a polyurea filler phase, and the PIPA process results in a urethane-dispersed phase. Copolymer polyols are commonly used in comfort applications such as seating where it is used to provide increased load bearing without having to increase overall foam density. As a result of the improved load bearing, the foam will also exhibit less shrinkage upon cooling due to its inherent ability to resist shrinkage stresses. In these roles, the combined copolymer polyol market was approximately 650,000 metric tons in 2010, growing at about the rate of the automotive industry. In use, the critical characteristics sought by customers is the percent polymer in the polyol (the solid level), viscosity of the filled polyol, filterability or polymer-phase uniformity, and final product characteristics such as color, VOC, and odor minimization. Virtually all producers of polyether polyols sold into comfort seating applications also offer filled polyol options to their customers.