Polyurethane Building Blocks
The versatility of polyurethanes is derived in large part from the wide selection of building blocks available to materials designers. The growth of polyurethanes has been highly dependent on the cheap and available feedstock polyisocyanates, polyols, and chain extender coreactants such as water, alcohols, and amines. However, this paradigm has recently been challenged as substitutions are made for purposes of avoiding health and safety issues associated with isocyanates. A detailed discussion of substitute chemistries to urethane structures will be handled in Chapter 12 [1-3]. This chapter provides an overview of building blocks for conventional polyurethane polymerization.
In principle, there is as much potential for design of isocyanate structures as there is for alcohol and amine coreactants. In reality, while there are numerous polyisocyanates to choose from, most of the innovation in polyurethane performance comes from the broad range of choices available in the coreactant alcohols and amines. To a great extent, this reflects complications (both industrial and regulatory) associated with making isocyanates and the comparative ease of making polyol and polyamine structures. It also reflects the fact that for most polyurethanes, nonisocyanate reactants comprise more than 50% of the polyurethane volume. Thus, for the purpose of obtaining any particular outcome, varying the polyol and the chain extender components is the most straightforward way to begin.
The term polyol refers simply to polymer backbones containing nominally two or more hydroxyl groups. Polyols are the largest volume raw material used in polyurethane applications with weight fractions in applications ranging from 90wt% in low modulus flexible sealants, 70wt% in flexible foams, and as low as 30wt% in rigid insulation foams. As implied by these values, the polyols in urethane formulations tend to provide softness and flexibility, while isocyanates and low-molecular-weight chain extenders provide hardness and stiffness to the resulting polymer structures.
Polyols are produced with a range of backbones and hydroxy functionalities that can be tailored to best meet application processing and property requirements. The most industrially significant polyol backbones are ether and ester based, while a number of specialty backbones including carbonate, acrylic, and ethers derived from tetrahydrofuran (THF) are used in high-performance coating, adhesive, and elastomer applications. Figure 2.1 shows their relative global volumes as of 2011 [4, 5]. For comparison, the relative global volume for polyether polyols in 1992 was 2654 thousand metric tons and for PTMEG was 100,000 metric tons . In the case of polyether polyols, this translates to roughly 4% annual growth rate. The volume growth of PTMEG has also been a long-term trend of approximately 4% . Despite continuity of the long-term trend, the growth in these categories has not been linear over time and not uniform over all geographies. For instance, it is estimated that while the current growth rate for polyether polyols in China is nearly 8%, it is only about 2.5% in North America. While the polyol industry as a whole has grown, the relative size of each category reflects the integrated underlying dependence on global economic growth in various regions.
FIGURE 2.1 Relative volumes of polyols produced in 2011.
Polyols vary markedly in structure, manufacture, function, and price. While each occupies a particular price/performance niche, for many of them, there is an overlap, and the choice of which polyol to use in any particular application depends on the history and experience of the chemist and the end user. In general, polyols all begin with low-cost commoditized building blocks. Final cost depends on factors related to volume of production, cost of polymerization process, and perceived value in an application. The structures of several commonly utilized polyols are provided in Figure 2.2. The structure to a great extent defines the properties of the resulting polyurethane, as well as the compromises each structure demands of the final product performance. For instance, while polyethers generally provide good low temperature performance, an easily processed backbone, and a good cost position, the low ceiling temperature (the temperature at which the rate of depolymerization is equal to the rate of polymerization) is relatively low for these structures . While many factors may influence the exact degradation temperature, these polyether polyols should not be considered stable above 220 °C. Polyether polyol riammability results from the high volatility of the monomers and the combination of oxygen and hydrocarbon fuel that the monomer represents and is a fundamental limitation to these structures [8,9]. This problem has in turn created a demand for flame retardants in many polyether containing systems, especially foam systems having high surface to volume ratios . Similarly, the well-known hydrolytic instability of polyesters is a compromise that must be accommodated if taking advantage of their low cost (in some instances) and high thermal stability (Table 2.1).