Spray, Poured, and Froth Foams
While isocyanurate boardstock rigid foams dominate the construction market, other modes of application find significant uses. The most common alternative application methods are called "pour-in-place" (PIP) and spray or froth spray. Both of these methods involve taking a formulated system of polyurethane building blocks, surfactants, catalysts, flame retardants, and so on, and performing the foaming at the construction site. Since the applicator is transporting a relatively dense liquid to then make very low-density foam, transportation costs are lower than with foam boardstock. Further, these modes can be used to apply foam over wide irregular areas in the case of spray, or to fill irregular cavities in the case of PIP.
Spray Foam Spray foams are used for the manufacture of insulated structures where coverage of large areas is needed, where insulation of complex shapes is required, and where coverage without joints or seams is required. The foam is applied though a mixing head/nozzle connected by hoses to two or more tanks containing the foam formulation. Since the foam must be applied quickly over wide areas, the formulation exits the application nozzle at a high velocity after high-pressure impingement mixing. Typical operating pressures are from 700 to 2000psi. Because of the need to flexibly deliver insulation from a central tank system of feedstocks to walls and roofs, hose lengths can be as long as 300 ft or more. Like many liquid delivery nozzles, the end of the nozzle has a fixture to deliver a specific pattern that can be adjusted and substituted. Foam delivered by a spray method is usually qualified by a battery of standardized tests including those listed in Table 8.1. Specific qualifying spray foam tests are a reactivity profile, limiting oxygen index for flame ignition, flammability testing, smoke density testing, and trimer content (by FTIR; see Chapter 5). The procedure for developing a spray foam formulation is not simple and can be laborious and highly iterative. Achieving a system that meets all qualifying tests simultaneously is a challenge. The task is usually broken into four parts from which preliminary information and indications of performance can be gleaned prior to large-scale expensive flammability testing. This developmental procedure is summarized in Figure 8.12.
The rigid spray foam system is relatively simple in concept (see Fig. 8.13), but there are myriad details that are difficult to convey in a picture. As with all polyurethane systems, the quality of what is obtained depends on strict control of stoichiometry, which in the case of spray systems requires careful calibration, proportioning, and control of liquid pumping systems. Further, temperature and mixing conditions can likewise strongly influence foam properties. Last, it is the author's experience that skill in handling the spray gun and choice of application pattern can also have a strong influence on insulation appearance and quality.
FIGURE 8.12 Typical process for developing a polyurethane spray foam formulation.
FIGURE 8.13 Schematic of the components for application of spray foam.
The formulation of foam will to some extent be dictated by application, building codes, and, increasingly, the building's insurer . For many spray foams applied for roof insulation, the flammability inhibition must be very high (Class 1 determined by UL-94 test for flame spread and smoke generation), which can dictate the use of a relatively large amount of an optimized flame retardance/smoke inhibiting additive package. Table 8.2 provides example formulations for spray foam roofing applications and the measured properties and performance of these formulations.
A specific polyol initiated from a Mannich base  is specified in the formulations of Table 8.2. This is a unique polyol used almost exclusively in spray foam applications. It is unique in that it is (i) autocatalytic (see Chapter 2), (ii) has high functionality and low equivalent weight, and (iii) is aromatic. These attributes contribute to the requirements of spray foam technology by (i) speeding the reaction to enable fast rise at low cure temperatures, (ii) increase reaction rate and decrease gel times to improve reaction speeds and allow application to vertical surfaces, and (iii) contribution to the required flame retardance of the overall system. Figure 8.14 details the synthesis and structure of the polyol. The Mannich polyol most established for the spray foam application is the reaction product of nonyl phenol, formaldehyde, and diethanolamine. Manufacturers of Mannich polyols provide formulation
TABLE 8.2 Formulation for producing spray foams
While the formulations are relatively different, the resulting foams only differ in some dimensions of performance showing that the formulation will be driven by the particular needs of the structure.
Flexibility by varying the ratio of formaldehyde, ethanolamine, and alkoxylate to provide a specific equivalent weight and functionality.
Froth Foams Spray foams can be more efficient than foam boards in some applications due to the inefficiencies associated with transportation of bulky low-density materials. However, in some circumstances, spray foams can also be inefficient. For instance, spray foam applications tend to be large volume and large surface area. For applications where there are many relatively small areas to apply foam, a product has been established providing needed product convenience and cost effectiveness.
FIGURE 8.14 Process for production of Manmeh polyols for use in spray foams.
Rather than the high pressure charging of materials in conventional spray foam combined in impingement mixing, the low-pressure application (called froth foam) product is charged at a relatively low pressure to a nozzle that has a static mixer contained within the nozzle handle . The mixed polyurethane streams exit the nozzle in a relatively lower volume targeted spray that might allow insulation in a small crawl space ceiling, around a window frame, to a section of piping, to a small vessel, or other place for a spot application of insulation. The two component mixture is called froth foam due to the formulation components being charged to their vessels with insulating gas blowing agent, and often a pad of inert gas such as nitrogen. The tank pressure is in the range of hundreds of psi, and this gas pressure is the motive for the movement of materials from the tanks to the spray gun. Flows are controlled by valves to make sure the relative proportions of the two components are at the levels specified by the manufacturer. As indicated, both components of the froth foam system are under pressure. As the gas/polyurethane system flows into the relatively lower pressure environment of the nozzle handle, gas bubbles will begin to evolve from the flowing streams creating the froth. The frothing streams must mix in the static mixer within the handle. While the forming bubbles create some agitation in the liquid, their formation does not necessarily facilitate mixing. In fact, foams are notoriously poor as a mixing medium [22-24], and the static mixer and handle design
FIGURE 8.15 Drawings of (left) the external view of a froth foam gun and nozzle and (right) cutaway showing the froth chamber and static mixer. Image Courtesy of Peter Schultz Dow Chemical Company.
FIGURE 8.16 Form (left) of a purchased froth foam system and function (right) of a froth system in use. (See insert for color representation of the figure.)
is a highly engineered device and an object of highly competitive and fiercely protected intellectual property. In fact, more than the formulations comprising the foam, the nozzle and the ergonomics of the froth foam packaging are the most intensely patented aspects of this technology [25-29].
While the nozzle design is highly protected as a marketing asset, there are certain basic features that make up a typical nozzle. Such a generic nozzle is pictured in Figure 8.15 and the basic form of the product is shown in Figure 8.16. The contents of the froth foam formulation are unique in that the foam does not depend on the chemical reactions that occur to make the polyurethane reduce density. While some froth foam formulations will have water, the levels are small, usually on the order of 1% or less. Instead the froth foam depends on the expansion of the pressurizing gas
TABLE 8.3 Formulations for froth foam that might be use as a window sealant (Formulation 1) and as insulation foam for an attic space (Formulation 2)
and the blowing agent to create a porous structure. Some exemplary formulations are presented in Table 8.3, and a comparison of typical spray foam versus froth foam properties is given in Table 8.4.
PIP Foams PIP rigid construction foams fill a particular application space that other less-expensive techniques cannot be modified to fill. PIP is a discontinuous foam production process that places foams in complex cavities less expensively than low-volume froth foam techniques, and much less expensively than highly capital-intensive continuous techniques [30, 31]. Like froth foams, the components are taken to the application site, mixed, and poured or sprayed into cavities. The polyurethane building block chemicals then foam to fill the cavities. The requirements of this process are unique but can overlap the other methods depending on the details of "the pour." One distinguishing feature of PIP is that the foaming and cure are somewhat slower than spray and froth applied foams since the foam must be able to flow into all the cavities
TABLE 8.4 Comparison of typical properties of high-pressure spray foam and froth foams
FIGURE 8.17 Pour-in-place application segments (nonappliance).
and irregularities. Foam thickness in PIP applications can be significantly larger than other rigid foam applications. The applications PIP applications methods find their largest uses in are shown in Figure 8.17.
The details of the cavity dictate the processing technique. The cavity filling technique must effect uniform distribution of the mixed and reacting polyurethane, optimize flow to all parts of the cavity, allow for displaced air to escape, eliminate entrapped air, minimize void formation, and minimize cell orientation (anisotropy) . Like high-pressure spray techniques, the two components must be mixed, and this is usually achieved using impingement techniques such as occur in spray polyurethane foam operations [33-35]. However, unlike spray foams, PIP exits the nozzle with a relatively slow forward velocity and with laminar flow. In addition, unlike spray foam, PIP operates with a controlled, usually programmed time or volume-controlled shot size. The foam is then injected into the cavity using procedures optimized for the particular shape. An alternative technique uses a lance withdrawal approach. In this method, a hollow wand directs the polyurethane components into the cavity which will be moved about the volume and withdrawn as the components are deposited. This technique minimizes the flow requirements of the dispensed foaming mixture and is useful for narrow and long cavities of moderate-to-lower volumes.
As mentioned earlier, relative to spray techniques, a discontinuous technique such as PIP requires slower reactivity. Relative to spray application that must gel in less than 10 s, PIP foams gel in 90-120 s for larger cavities, and 30-50 s for smaller cavities such as entry doors. The ideal foam insertion initiates volume expansion after completion of liquid injection to minimize pouring (and concomitant partial distortion and collapse) into a rising foam. The reacting foam should gel shortly after rise to minimize cell anisotropy (stretching in the rise direction) and resulting weakening of the foam. Last, the PIP foam must fill the space intended, but not deform the object containing it. For example, when poured into a window frame, a hollow entry door, a reefer container, or a boat's hull cavity, it must not deform the object containing it due to overexpansion of the foam by overfilling, or to overpressure of the blowing gas. Such perfect performance is not usually realized but is approached by the appropriate formulation choices. Table 8.5 provides example formulations of PIP foams for different uses. The tlameretardant requirements are often less for PIP applications than for wall and roof insulations that have a larger surface, whereas PIP foams are encased in a containing envelope that also protects the foam from an initial ignition source .
Water-blown PIP technologies exist in the market place, and in some cases can have a price advantage over new generation tluorocarbon-blowing agent formulations due to the high prices of blowing agents. Water-blown PIP can be disadvantaged by having significantly longer cure times, higher densities, and higher K values (lower R values— poorer insulation properties). In addition, they can often have significantly higher viscosity that complicates the efficiency of mixing and nozzle delivery. Of course, blowing agent formulations along with being more expensive also require the use of pressurized tanks to maintain the system sides in a liquid state.