Results and Discussion
As expected, at the low temperature, the corners are pulled inward, resulting in a bending angle greater than 90°; whereas at the high temperature, the comers are pushed outward, resulting in a bending angle less than 90°. Figure 5 shows, as an example, the deformed comer shapes for the aluminum spacer at the low and high temperatures exaggerated by a factor of 100.
Simultaneously, the primary seal (PIB) cross section decreases at the low temperature and increases at the high temperature. Figure 6 shows the “pumping” effect occurring in the PIB primary seal as a result of the thermal cycling. The aluminum spacer, having a larger coefficient of thermal expansion (c. t.e.), com-
Cold (-УГС) Ambient<+23“C) H«I*WC)
FIG. 6—Pumping of PIB primary seal as a result of thermal cycling.
bined with the sealant (DOW CORNING 982) having a high modulus and larger coefficient of thermal expansion, has the greatest effect on the corner of the IGU resulting in a large change in cross-sectional area during the change from hot to cold. Monitoring the changes occurring in PIB primary seal thickness around the circumference of the IGU, the authors found that the stainless steel spacer had, by far, the least effect on the change in cross-sectional area, while the aluminum spacer had the largest effect, see Table 1.
The spacer design and the material choices (spacer, sealant) also have a pronounced effect on the nominal strain distribution in the edge-seal. Figures 7-9 show the nominal strain distribution in the edge seal for the aluminum, galvanized steel, and stainless steel spacers at the cold (-30°C) and hot (+60°C) temperatures.
As can be seen, the maximum strain for the aluminum and galvanized steel spacers occurs in the primary seal region, while the stainless steel spacer results in a more even strain distribution. This finding is in keeping with the expected performance based on the difference in thermal expansion coefficients between spacer material and float glass. Thus, changes in the effective cross-sectional area of the primary seal available for diffusion that arise from differential thermal movements, are likely to account for the observed performance differences of IGUs having different spacer materials.
TABLE l—Deformation of PIB primary seal as a function of spacer material