Virtually, all urethane-based boardstock used in the construction industry, especially when used for roofing or walls is based on polyisocyanurate chemistry . The formation of isocyanurate structures is covered in
Chapter 3. A high functionality polymeric MDI is usually employed. It is used in large stoichiometric excess in the overall foam formulation to facilitate reaction of one isocyanate with another. Isocyanate reaction to form ring structures is further promoted by employing catalysts known for specifically forming isocyanurate rings as discussed in detail in Chapter 3. These catalysts are usually carboxylate salts or amine salts. Examples are, for instance, potassium octoate and the 2-ethylhexoate salt of 2-hydroxypropyltrimethyl ammonium. The catalysts act by strongly complexing the electropositive isocyanate carbon inducing a high enough negative charge at the isocyanate nitrogen to complex the carbon of another isocyanate propagating to an oligomer of 3 [11-13]. At that point, the six-member ring closes to form the ring structure shown in Figure 8.7.
The isocyanurate structure is particularly well suited to construction applications due to its inherently greater flame retardancy and thermal stability than possessed by urethane or urea crosslinks . In addition, isocyanurate's high crosslink potential through structure (Fig. 8.7) creates a more rigid board structure. The high isocyanate content used in the formulations further creates the potential for very good adhesion to other components of a composite structure such as might be employed to make a sandwich panel. Such composite materials could be aluminum or steel facers and fiber glass mats for additional dimensional stability and fire performance [5, 15].
Table 8.1 provides representative formulations for isocyanurate polyurethane rigid foams. Formulation 1 is what might be well used in a commercial building and can be exposed to the building interior by virtue of the special design for flame retardance by using high levels of isocyanurate and added flame retardant. Formulation 2 is what might be utilized in residential insulation applications for external wall cavities. An aluminum-faced isocyanurate foam image is provided in Figure 8.8 showing the closed cells and the very thin aluminum facing employed for improved time dependence of the insulation properties and improved flame retardance.
The flexural modulus is also a measured property of the insulation foam composite structure. While the ultimate properties (i.e., strength and elongation at
FIGURE 8.7 Trimerization of isocyanate functionality to form an isocyanurate ring. For rigid foams the R group would include additional unreacted isocyanate functionality capable of further reaction during polymerization.
FIGURE 8.8 Photograph of an isocyanurate boardstock and scanning electron microscope image of the isocyanurate foam and the adhered aluminum film cover.
TABLE 8.1 Isocyanurate foam formulations and properties for making PIR insulation boardstock
break) are useful for predicting conditions for catastrophic structural failure, the low-strain properties are also useful for predicting the applicability of a structure for a specific application. As an isocyanurate sandwich board is a large composite structure, the flexural modulus can provide this kind of practical information . The equation for the modulus of a bending beam (as measured by a three-point bend test) is given by Equation 8.1 where L is the beam length, b is the beam width, h is the beam height, and m is the slope of the linear elastic portion of the load deflection curve.
In the case of an isocyanurate foam composite structure with the length and width of the structure held constant, the thickness of the insulation provides the main variable for controlling structural properties prior to failure. With the modulus varying as the inverse cubic power of thickness, it is seen that that foam thickness dominates the properties of the structure (Fig. 8.9) with small inclusions and flaws being minor perturbations.
Another property that the builder may try to influence is the overall insulation of a given structure. As Figure 8.10 shows, it is again the board thickness that most effects the expected energy conservation that can be designed in with the thickness-insulation relationship being linear. Isocyanurate foam (also called "iso" or "PIR" foam) is typically available in thicknesses from 0.5 to 2.375 in. They are made in plant manufacturing operations such as illustrated in (albeit simplified) Figure 8.11. The operation is complicated by the numerous processes
FIGURE 8.9 Measured flexural moduli of isocyanurate board properties using 6-in. long 2 in. wide samples with deflection rate or 0.5 in./min.
FIGURE 8.10 Measured lvalue (ASTM C518) as a function of board thickness for formulations 1 and 2 of Table 8.1.
FIGURE 8.11 Schematic of the process for making isocyanurate boardstock. A third roll is sometimes installed with glass matt that gets laid directly into the foaming mass.
that are occurring simultaneously from the application of the reactants to the moving conveyor, the foam rise, the application of the laminating films, curing of the foam, development of adhesion to the film, and movement of the finished boards away from the process location. Economic efficiency requires the plant be used at high utilization and at high linear rates [17, 18]. Linear board rates of 60m/min are possible;
however, this feat of engineering optimization can still be economically disadvantaged if the plant capacity is not run at or near maximum utilization, since there are other insulations, that although less effective, may still be price advantaged since isocyanurate insulation is the most expensive insulation product commonly available. Isocyanurate foam may be delivered a commercial advantage if the results of California State Assembly Bill 127, signed into law in September 2013, results in further tightening of building flammability standards.