Appliance foams have similarities to PIP foams in that they are used to fill a cavity and that they must not deform the cavity during foam rise due to expansion pressures or during contraction as the foam cools. The majority of polyurethane appliance foams are used for refrigeration including refrigerators, freezers, coolers, refrigerated trucks, and railcars . Another category is used for water heaters. Figure 8.20 breaks down the approximate volume ratios that occupy these two market segments. While there is a continuing demand for consumer appliances, the aggregate demand follows the construction market and particularly new home construction. Commercial trends in these markets relate to consumers obtaining second refrigerators and freezers, and growing use of refrigerated vending machines and display cases .
The typical polyurethane components of appliance foams are also similar to PIP foams with the notable exception that appliance foams are expected to provide little in the way of flame retardance relative to construction foams, or even flexible foams used in furniture. What they are expected to provide is (i) the ability to rise and cure rapidly with no deformation of the refrigerator or water heater cabinet, and
(ii) a high degree of insulation performance. Along with energy savings to the customer, highly efficient insulation allows the manufacturer to specify a smaller, less-expensive compressor for refrigeration to maintain a larger capacity. Optimum design can also help make up for poorer insulation efficiency arising from the use of environment-friendly, if less efficient, blowing agent/insulating gases. A discussion of insulating gases comes later in this chapter. Table 8.7 provides example appliance foam formulations.
The data given in Table 8.7 are actual for the foam formulations given, but the properties should be viewed as representative and illustrative of the fact that blowing agents are a major component of the formulation, and can have a major influence on the formulation and the properties. Further, different compartments of an insulating unit (i.e. the freezer cabinet vs. the freezer door) may require more or less efficiency, and the manufacturer may inject more than one foam formulation to optimize performance and price.
Table 8.8 provides a general guide to property requirements for this segment. Properties achieved with any particular formulation are variable and will depend greatly on numerous process specifics including the details of the injection, the shape of the mold to be filled, the ambient temperature, even the geographic elevation of the manufacturer making the insulation foam. While the properties may vary, the general trend in formulation optimization is indicated in Table 8.8 as designers continuously try to enhance foam efficiency along all performance dimensions.
While there are ASTM tests for measurement of thermal insulation panel performance such as ASTM C518, in the case of polyurethane rigid foams for appliance applications, the manufacturer will often rely on internal tests of performance. Rather than a screening test on the foam component, the test will incorporate the entire molded and foamed cabinet. This makes sense since energy leakage from, or into, a container such as a refrigerator may depend on the envelope materials and the overall design of the cabinet. Many of these tests are performed to conform to regulatory agency mandates requiring explicit evaluation and communication with consumers about expected energy consumption. An example of such protocols is the US Department of Energy "Uniform Test Method for Measuring the Energy Consumption of Electric Refrigerators and Electric Refrigerator-Freezers" 10 CFR Appendix Al to subpart B of part 430. A screening test for this battery of standardized tests and conditions is the test outlined by the Association of Home Appliance Manufacturers (AHAMs) test referred to as the "reverse heat leakage" (RHL) test. The RHL test is performed on a complete cabinet without an installed compressor. A light bulb and energizing circuit is placed inside the insulated cabinet that is then placed in a cold room (Fig. 8.21). The light bulb is energized and the amount of energy required maintaining the interior at an elevated temperature is recorded. While still a bit involved and requiring a high level of control, this is a relatively low-cost method to screen the thermal efficiency of a foam-insulated appliance. The overall refrigerator conductance (Rc) is calculated from this experiment using Equation 8.2, where the temperature is usually an average of several measurements .
TABLE 8.7 Formulations and properties for use in appliance insulation applications
TABLE 8.8 Typical range of rigid polyurethane foams for appliances and the direction foam designers are trying to improve their foams in that dimension
FIGURE 8.21 Illustration of the concepts in a reverse heat loss test for refrigeration insulation efficiency.
The reverse heat loss measurement correlates very strongly with the thermal conductivity of the blowing agent insulating gas employed for making the rigid foam as shown for a controlled data set (Fig. 8.22) .
Figure 8.22 shows the importance of the blowing agent on the insulation performance and on the manufacturer's ability to meet regulator and customer expectation. In fact, as will be shown later, blowing agent technology and associated
FIGURE 8.22 Relationship between measured blowing agent thermal conductivity and the results for reverse heat loss test using foams blown with the same blowing agents.
formulation adjustments make up the field of most intense patent activity for the large polyurethane building block providers despite the fact that none of them is a blowing agent manufacturer.