Quantification of Differential Thermal Movement in Insulating Glass Edge Seals Using Finite Element Analysis

ABSTRACT: 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. Using finite element analysis (FEA) the authors modeled the thermal movements occurring in the edge seal of a large IGU (1.5×2.1 m2) as a result of temperature variations (-30°C to +60°C) for three commercially available spacer bars of different material and design. The model was based on nylon corner keys for the aluminum and galvanized steel spacers and bent corners for the stainless steel spacers. The nylon corner keys were assumed to be solid and firmly bonded to the spacers; whereas the bent corners were modeled as solid, bent metal corner keys, also firmly bonded to the spacers. Since actual bent corners are hollow, the model tends to overestimate the stresses for this corner design. As expected, at the low temperature, the corners are pulled inward, resulting in a bending angle >90°; while at the high temperature, the corners are pushed outwards, resulting in a bending angle <90°. Monitoring the changes occurring in the thickness of the polyisobutylene primary seal 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, while 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.

KEYWORDS: Insulating glass unit, differential thermal movement, spacer, finite element analysis

Introduction

Insulating glass units (IGUs) are exposed to a variety of environmental factors, such as temperature and atmospheric pressure fluctuations, wind loads, working loads, sunlight, water, and water vapor that nega­tively affects their service life [1]. During service, the edge seal of the glazed IGU is exposed to a microclimate within the window frame or curtain-wall construction that strongly deviates from the ambient climate. Two major studies have been conducted in an effort to monitor this microclimate in terms of edge-seal temperature, moisture, and presence of liquid water over a period of several years [2,3]. Whereas in Central Europe edge-seal temperatures of clear glass IGUs seldom exceed 40-50°C, for tinted or coated glass units or in warm climates service temperatures may well reach 80°C and above for prolonged periods of time [4].

In order to withstand these environmental loads, an IGU edge seal must have the following properties:

• Durability, i. e., resistance against environmental factors (both in terms of physical properties and adhesion).

• Structural strength that constrains movement in the edge-seal to minimize changes in the effec­tive cross-sectional area of the primary seal available for diffusion.

• Low moisture – and gas-permeability under service conditions.

Differential thermal movement between the spacer and the glass panes is a key contributor to the aging of the insulating glass edge seal and of the IGU itself. Repetitive shear and tensile cycling induces a pumping effect in the polyisobutylene (PIB) primary seal that over time displaces the primary seal and

FIG. 1—Glass pane element modeled.

generates voids, resulting in an increased leakage rate of the IGU. 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. Therefore, a quantitative evaluation of the magnitude of differential thermal movements in an IGU edge-seal configu­ration is essential in predicting the service life of an IGU.