In application to polyurethanes, polycarbonate polyols are among the highest performance polyol backbones available in the marketplace [65, 66]. They are also among the most expensive and produced at relatively small volumes. The world capacity for polycarbonate production is approximately 60 million pounds and consumption in 2010 was approximately 50 MM lbs . Relative to other polyols, the properties most attractive about polycarbonate polyols are thermal, UV, and hydrolytic stability. This suite of attributes has resulted in polycarbonate polyols finding their widest application in very-high-performance applications of coatings, elastomers, and adhesives.
Preparation of Polycarbonate Polyols The preparation of polycarbonate polyols is straightforward in its simplest form. It is the reaction of phosgene or a carbonate monomer, usually dimethyl carbonate (DMC) with a diol monomer or a mixture of diol monomers (Fig. 2.29). In the case of the phosgene route, the liberated hydrochloric acid (HC1) is usually quenched with NaOH to form the filterable salt. This route is generally discouraged due to the significant environmental health and safety drawbacks associated with large-scale handling of phosgene.
DMC is a relatively more attractive reagent for polycarbonate synthesis because of its low molecular weight, its relatively high unit ratio in the polymerization, and its relatively low cost and ready commercial availability. One disadvantage of using DMC is the evolution of toxic methanol requiring safety protocols in production. A more significant issue is the 30/70 DMC/methanol azeotrope that forms resulting in lost DMC in the condensation product [67, 68]. The practical effect is to make usage of DMC quite inefficient since the methanol/DMC azeotrope is difficult to separate by normal methods . One approach is to determine the minimum in the
FIGURE 2.29 Preparation of polycarbonate polyols from phosgene (not recommended) or dimethyl carbonate.
FIGURE 2.30 Preparation of polycarbonate polyols using an elevated pressure procedure.
DMC/MeOH concentration in the azeotrope as a function of temperature. The reaction is then run controlling the reaction vessel to the temperature, producing overhead products at the minimum of DMC in the azeotrope. However, this approach can be unworkable from a practical standpoint since the boiling point of DMC is about 90 °C, a temperature at which the reaction rate between the diol and the carbonate is very low. Thus, one efficient procedure for preparation of polycarbonate polyols using DMC is to run the reaction vessel at the optimum temperature for reaction, which at the same time optimizes the use of DMC by minimizing its content in the azeotrope-collected overhead. This can be accomplished by slow feeding ("starve feeding") the DMC to the molten rapidly mixing diol. The optimum overheads temperature is about 58 °C. The exact conditions for this will depend on the geometry of the reaction vessel, the control of reaction vessel heaters, and the mixing of the reaction vessel.
An alternative efficient method for producing polycarbonate polyols is to perform the reaction at elevated pressure. The reaction is performed in an autoclave with all components of reaction present (Fig. 2.30). The temperature and pressure are elevated with the purpose of driving the reaction to a stage at which a large amount of DMC has been converted to carbonate diol adduct. Subsequently, the pressure and temperature are reduced to drive the reaction to completion by removal of methanol. While still inefficient in DMC, this method provides improved time to a final product and does so in a smaller production footprint .
An alternative polycarbonate polyol is the polyalkylene carbonate formed from the reaction of oxiranes and C02. A common problem associated with this synthesis has been the formation of five member cyclic carbonates. There has been recent
FIGURE 2.31 Preparation of polycarbonate polyol using an alkoxylate.
activity with new catalysts that have proven themselves able to result in high-molecular-weight polycarbonate polyols (185-250 kDa) and low-molecular-weight polyols useful in PU synthesis (Fig. 2.31) [71, 72]. Despite the low cost of the building blocks, pricing of these materials is approximately the same as those achieved via condensation techniques described earlier.
There are numerous catalysts for this polymerization, heterogeneous and homogeneous, varying by efficiency, tendency to form the undesired cyclic carbonate, and resulting tacticity of the final polyol product. The tacticity is one of several controls of polyol structure that can be used to design final properties of polypropylene carbonate polyols (Fig. 2.32).
Design of polycarbonate polyol structure is primarily a matter of choice of which diol or mixture of polyols to react with the carbonate linkage. Designed artfully, the properties of the resulting polycarbonate polyol can be varied widely and have considerable effect on a derived polyurethane property. Table 2.9 is a list of diol monomers polymerized with DMC to make polycarbonate polyols, typical applications, and the physical state of the polyol produced. Hexanediol is a preferred monomer due to its reduced tendency to volatilize during polymerization (compared to 1,4-butanediol). It is also apparent that diol additives that randomize or disrupt regular chain structure help make the polyol liquid.