Acrylic polyols differ in numerous ways from the polyols described in the previous text, but they are an important and common occurrence in our day-to-day experience. Unlike the previously described polyols, which we may find ourselves sitting upon, walking upon, wearing, and depending upon in numerous but silent functions, acrylic polyols in isocyanate formulations are generally limited to coatings and particularly, but not exclusively, to automotive surface finishes [73-75]. In addition, acrylic polyols find application in wood coatings and anticorrosion finishes. Acrylic polyols are preferred for achieving higher hardness, UV stability, and water and acid stability. In contrast, polyester
FIGURE 2.32 Polycarbonatepolyols from alkoxylates and the structural variation that can manifest due to addition mechanism.
polyols are preferred in the same applications where solvent resistance, toughness, and flexibility are critical. Polyesters may find application for the same applications where hydrolytic stability is less important and cost is paramount. As commonly used, almost 200 thousand metric tons (440 million pounds) of acrylic polyols were used worldwide in 2012, with an average annual growth rate of over 4%, tracking the overall use of urethane surface coatings [76, 77]. As in all of these numbers, the use and growth rates are not globally homogeneous.
Preparation of Acrylic Polyols
Acrylic polyols are prepared like many acrylics, via radical initiation. Any acrylic monomer may be used as long as hydroxyl functionalized monomers such hydroxyethyl methacrylate, hydroxy butylacrylate, hydroxyethyl acrylate, etc. are present in the backbone . The role of any particular
TABLE 2.9 Select polycarbonate polyols (based on diol variation) and the difference in properties
acrylic monomer in the preparation of an acrylic polyol would be the same as in any acrylic resin: to adjust glass transition temperature, hardness, flexibility, or barrier properties. Styrene is also commonly used in acyclic polyol synthesis to increase hardness. Polar nonhydroxy components such as acrylic acid and acrylonitrile can be used to increase interchain polar interactions and so increase the tensile strength and ultimate elongation of a resin at the potential cost of decreasing low temperature flexibility . Allyl alcohol has been employed as a monomer for modification of molecular weight. Depending on the application, the molecular weight of acrylic polyols can vary from under a thousand to several thousand with hydroxyl equivalent weight from several hundred to several thousand. To maintain control of the polymerization process and maximize conversion of monomer to polymer, the polymerization is usually performed in a solvent such as ethyl acetate or an aromatic such as xylenes. The solvent can then be removed, or be used as a solvent for subsequent application of the coating. The carrier solvent aspect of acrylic polyols has become an area for technological innovation as producers try to limit solvent content and reduce regulated volatile organic carbon (VOC) emissions. Common initiators for acrylic polymerization such as peroxides are also commonly used in this process. An illustrative formulation and polymerization product is shown in Figure 2.33. In an actual formulation, the butyl acrylate and styrene could be balanced to provide the desired level of flexibility and hardness for the given application. The amount of hydroxyl functionalized acrylate would be balanced to provide the desired cross-link density for the coating.
FIGURE 2.33 Example preparation of an acrylic polyols.
Acrylic polyols have also been designed to optimize compatibility with aqueous systems. This is normally done by including acrylic acid among the monomers . The acid is subsequently neutralized with an amine such as dimethyl ethanolamine and water. The ratio of monomers will control the final properties of the film as well as the differential solubility.