Summary and Conclusions
Differential thermal movement between the spacer frame and the glass panes is a key contributor to the aging of insulating glass edge seal and of the insulating glass unit (IGU) itself. Repetitive shear and tensile cycling induces a pumping effect in the polyisobutylene (PIB) primary seal. This pumping effect may over time displace the primary seal and generate voids, resulting in an increased leakage rate of the IGU, as has been observed on IG units exposed to accelerated testing or in-service conditions (see references cited in
 ). Depending on the physical properties of the secondary seal, the mechanical cycling may also induce fatigue aging in the edge seal. Finally, the differential thermal movement also affects the opening of the primary and secondary seals and, therefore, the effective cross section through which diffusion of water vapor and fill gases occurs.
FIG. 9—Nominal strain distribution in edge seal (stainless steel spacer, DOW CORNING 3-0117).
The FEA modeling performed by the authors confirmed that, as expected, at the low temperature, the corners of the spacer frame are pulled inward, resulting in a bending angle greater than 90°; whereas at the high temperature, the corners are pushed outward, resulting in a bending angle less than 90°. Monitoring the changes occurring in PIB primary seal thickness along the circumference of the IGU, the authors found that the stainless steel spacer had, by far, the least effect on the change in the cross-sectional area, whereas the aluminum spacer had the most substantial effect. 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.