Aliphatic Polyester Polyols
Many aliphatic polyester polyols have been prepared and at one time or another been offered on the market. Despite this, and by far, the largest volume aliphatic polyester polyol used in polyurethane applications is polybutylene adipate followed by polycaprolactone. The ratio of polybutylene adipate to polycaprolactone production is approximately 100:1 . The properties of the polyester polyol are of course a function of the diacid and diol building blocks used to produce the polymer. The properties of the polyester polyol are most easily accessed by their thermal properties. Table 2.4 provides a limited view of the thermal properties based on the polyester components.
A simple analysis of trends in the polylactone sequence shows that the carbon length between ester linkages is strongly influential on the crystalline melting point and the crystalline melt enthalpy, asymptotically approaching that of polyethylene (Fig. 2.24). For polyesters prepared from diacids and diols, a simple analysis of crystalline melting point and enthalpy shows virtually no correlation relative to the length of the diol, with a square of the correlation coefficient (r2 = 0.1 or less). With the exception of polybutylene succinate, there is a positive correlation of increasing diacid length with melting point and melt enthalpy (r2=0.8 and 0.5, respectively).
Aromatic Polyester Polyols
Aromatic polyester polyols are primarily used in the production of polyurethane and polyisocyanurate foams for construction and appliance applications. There are additional applications in slabstock, spray foams, elastomers, and adhesives. Commercially, aromatic polyester polyols are based on diethylene glycol or ethylene glycol and low-molecular-weight PEGs, polymerized with either dimethyl terephthalate, terephthalic acid, phthalic anhydride, or crude recycle scrap of polyester terephthalate (PETr) . In some cases, polyol functionality is increased by addition of trimethylolpropane and pentaerythritol. Of
TABLE 2.4 Properties of selected polyester polyols useful in polyurethane synthesis and comparison to high-density polyethylene
FIGURE 2.24 (a) Melting point and enthalpy of melting of polyester polyols derived from lactone ring opening polymerization as a function of ring size. Properties asymptotically approach those of polyethylene, (b) Melting point and enthalpy of melting for polyester polyols made from condensation polymerization as a function of carbons in the diacid. The trends are somewhat weaker than for the lactone series.
TABLE 2.5 Comparison of polyols used for rigid appliance foam and rigid construction foam
Along with differences associated with gel time optimization, construction foams must also meet stringent flammability requirements.
course, any combination of aromatic and aliphatic monomer could be used for making these materials, but price and performance constraints also limit the menu of employable building blocks. The nonnegotiable and established performance and process attributes coupled to high price competition have led to commoditization of this technology .
Although the functions of construction and appliance insulation foams overlap, the details differ in some important ways and result in divisions in polyol specifications unique to the two markets. For the appliance market, aromatic polyester polyols are expected to contribute positively to maximizing thermal insulation, minimizing demold postexpansion, and maintaining structural integrity . These attributes are detailed in Table 2.5.
The overlap is significant only in those specific applications where the requirements are the same. In spray foams, it is imperative that the sprayed components foam and build strength very quickly, in a manner similar to that required for appliance foams but for very different reasons. In the appliance sector, there is criticality in being able to process more refrigerators, water heaters, or dishwashers in a shorter time. Also important to appliance manufacturers is that the foam blown into an appliance mold builds strength quickly to prevent deformation from the growing insulation internal gas pressure and not deform from shrinkage upon cooling. For spray foams, designed rapid strength development is a reflection of the desirability that the foam stick to vertical surfaces when necessary.
A critical requirement of construction foams is their flame retardance. It is not expected that the aromatic polyester polyol supply all of this property. The aromatic isocyanate and materials specifically employed to inhibit fire are also employed [57, 58]. However, fire retardance is considered a desirable feature of an aromatic polyester polyol in the construction foam market. The need to spray foam over distances also can limit the foam viscosity of a formulation that a specific application can tolerate. This can in turn dictate the viscosity (and so the structure) of the aromatic polyester polyol used in this application . It also requires in many cases that spray foam applications use significant amounts of low-viscosity polyether polyols in combination with polyester polyols to meet viscosity requirements. Terephthalic acid-based polyester polyols have an increased tendency to crystallize, so using TA-based
TABLE 2.6 Example of an aromatic polyester polyol for construction application
TABLE 2.7 Raw materials and unit ratios for two commercial aromatic polyester polyol formulations
polyesters may be limited by application temperature or may require polyol design considerations that inhibit crystal formation. Table 2.6 is an example of a commercial aromatic polyester polyol available on the market.
Unlike aliphatic polyester polyols, the synthesis of an aromatic polyester polyol may not follow principles for obtaining a defined product of condensation polymerization, but may instead target the properties required for a result. The final product may constitute a designed amount of free DEG, for instance, to obtain the required viscosity or reactivity.
Table 2.7 shows the unit ratios for two commercial aromatic polyester polyols. They are, in principle, the proportions for a diacid reacted with two diols. However, as actually practiced, the desired structure cannot be the calculated structure since polyester product represents an equilibrium, with structures from oligomers to free glycols and acids. The actual distribution of components can be calculated using a Flory-Schulz distribution or measured chromatographically. These methods are usually in good agreement . To meet required properties, it is common to adjust the final polyol with free glycol. In fact, between the two polyols described in Table 2.7, the substantial difference in viscosity is a function of (1) the lower unit ratio of diacid resulting in fewer ester linkages but primarily (2) the higher volume of free glycol in the final product [61, 62].
Since the aromatic polyester polyols are typically lower in molecular weight than aliphatic polyester polyols, there is a higher probability of finding free monomer in the final product. The free monomers can have a significant effect on the efficacy of the polyol in the final foam product. Product polydispersity in polyester synthesis is an expected phenomenon, but it is wise to measure, or at least calculate, the final product oligomer distribution. This is most easily done using the gamma distribution, or more commonly the Flory-Schulz distribution. The distribution is easily calculated based on knowledge of inputs such as the molecular weight of the repeat unit. The weight average of the Flory-Schulz distribution is calculated from Equation 2.8:
where Wx= (weight of all oligomers of weight x/weight of all oligomers) and p is essentially the fraction of converted reactive groups. While the number of monomers present as a fraction of total product weight may be small, their number may be relatively large even at fairly high monomer conversion to product. The number of oligomers as a function of conversion is calculated from Equation 2.9
The meaning of this distribution is shown in Figure 2.25, where it is seen that except at very high conversion, the mole fraction of monomers in the product polyol can be significant, even when their weight fraction can be quite small. Given the very low equivalent weight, their role in the final polyurethane properties can be important. The figures also illustrate the importance of high conversion percentages in obtaining high-molecular-weight polymers.
The free monomer diol components have a clear and unambiguous role in these polyols. They produce reactive components for the aromatic diacid of the polyol and for the isocyanate component of the subsequent polyurethane foam formulation. Additionally, they provide rheological tuning to the polyols. Beyond that, they also provide a means of improving compatibility of the components of the formulation. Lastly, as the cost of the diols is usually lower than that of the diacid, the diols can provide a means of managing overall product cost. Artful combinations of diols can, in principle, provide a means of performance innovation by improving properties at reduced cost.
The diacid components, terephthalic acid (or dimethyl terephthalate), and phthalic anhydride have significant effects on cost as well (isophthalic acid is rarely used due to cost considerations). In turn, they also exert a subtle effect on properties due to their different structures. Cost is affected by the so-called unit ratio of the building block—essentially a determination of mass equivalence for the different molecules within an equivalent final product. Although all final products are either identical (using terephthalic acid or dimethyl terephthalate) or isomeric (using phthalic anhydride), the differences relate to the molecular weight of the building block diacid and the mass of condensation product produced (Fig. 2.26). By this measure, phthalic anhydride is the most cost-effective aromatic unit; however, it is usually almost 20% more expensive than terephthalic acid resulting in supply issues usually being the deciding factor for which is used.
In addition, there are differences between the terephthalate residues and orthophthalic residues from phthalic anhydride (Fig. 2.27). The substitution pattern has a
FIGURE 2.25 Weight and number average distributions of oligomer sizes present in a polyester synthesis based on the Flory-Schultz distribution. (See insert for color representation of the Figure .)
FIGURE 2.26 Unit ratios for diacids or diesters used in preparation of aromatic polyester polyol synthesis.
FIGURE 2.27 Production of terephthahc acid and phthahc anhydride.
FIGURE 2.28 Comparison of lowest energy conformations of aromatic polyester polyol from (a) terephthalic acid and (b) orthophthalic acid (or phthalic anhydride) and DEG. The free volume requirement of the ortho positional substitution is much higher which may account for much of the viscosity difference between polyols made with the different co-monomers. (See insert for color representation of the Figure .)
substantial effect on free volume requirements of the polyol component, which in turn has an effect on subsequent Theological properties. Figure 2.28 shows energy minimized structures for the aromatic polyester polyol of DEG with terephthalic (a) and (b) orthophthalic acid with approximately equal energies accounted for by hydrogen-bonding interactions and overall conformational energies. It is clear that the aromatic polyester polyol from orthophthalic acid requires additional free volume due to the pendant benzylic residue but in turn allows more intramolecular van der Waals interaction between ether moieties. While the aromatic positional isomer is known to affect foam properties, it has not previously been associated with fundamental chain physics, and the causal relationship has been lacking .
TABLE 2.8 Properties of rigid polyurethane foams with aromatic polyester polyols having either terephthalic acid or phthalic diacid comonomers reacted with DEG
Viscosity data refers to the viscosity of the polyols used in the formulations. Other foam formulation factors were held constant.
Lastly, aromatic positional isomers are also associated with small but real differences in performance (Table 2.8). While the origins of these differences are not established, it emphasizes the role of positional isomers in establishing structure/ property relationships .