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

Transcription

1 James E. Stewart, 1 William R. O Brien, 2 and Andreas T. Wolf 3 Journal of ASTM International, Vol. 3, No. 8 Paper ID JAI13121 Available online at 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 m 2 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 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 Manuscript received May 5, 2005; accepted for publication May 4, 2006; published online June Presented at ASTM Symposium on Durability of Building and Construction Sealants and Adhesives on June 2005 in Reno, NV; A. T. Wolf, Guest Editor. 1 Senior Engineer, Dow Corning Corporation, Midland, MI Marketing Specialist, Dow Corning Corporation, Midland, MI Scientist, Dow Corning GmbH, 65201Wiesbaden, Germany. Copyright 2006 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA

2 2 JOURNAL OF ASTM INTERNATIONAL 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. Finite Element Analysis (FEA) Evaluation Using finite element analysis FEA the authors modeled the thermal movements occurring in the edge seal of a large IGU as a result of temperature variations for three commercially available spacer bars of different material and design. Modeling Considerations An IGU window size of 1.49 m 58.8 in. by 2.13 m 84 in. was chosen for the FEA modeling. This size simulates a large sliding glass door, with the same aspect ratio as the 0.35 m 14 in. by 0.50 m 20 in. test panes. Taking advantage of symmetry, the model needs only to take one quarter of the full size into account. In the model, the pane is supported along the bottom perimeter edge as shown by the arrows in Fig. 1. Element nodes on symmetry planes are constrained to remain on the symmetry planes. Galvanized steel and aluminum spacers are modeled with dimensions 12.3 mm in. wide and 8mm in. deep and a wall thickness of 0.4 mm in. for aluminum and 0.5 mm in. for galvanized steel. Corner keys for galvanized steel and aluminum spacers are modeled as solid polyamide Nylon 6 keys that are bonded to the spacers the model does not allow for any slippage between the spacer and the corner keys. Figure 2 shows the spacer and corner key designs and dimensions chosen. The stainless steel spacer is modeled with dimensions of 11.5 mm in. wide and 6 mm in. deep with a wall thickness of 0.2 mm 0.01 in.. The corner key is modeled as bonded to the spacers and as a solid. The actual corner key is hollow and the metal is split at the inner radius, therefore the stress predicted by the model is much higher than the actual stress expected in service. Figure 3 shows the stainless steel spacer and corner key designs and dimensions chosen. The edge seal of the galvanized steel and aluminum spacer is modeled with the secondary seal constrained by bonding to the spacer and the glass panes, but not to the nylon corner key, reflecting the fact that secondary sealants generally have poor adhesion to plastic corner keys. For the stainless steel spacer, the edge seal is modeled with the secondary seal constrained by bonding to the glass panes and the spacer, as was the case for the galvanized steel and aluminum spacers. However, the corner keys in this instance are bonded to the secondary seal as well as the edges in contrast to that described for the other two spacer types. The secondary sealant thickness measured as coverage above the spacer is assumed as 6 mm. The glass thickness is modeled as 6 mm in. and a 0.3 mm in. gap is assumed for the primary

3 STEWART ET AL. ON DIFFERENTIAL THERMAL MOVEMENT 3 FIG. 2 Spacer (aluminum and galvanized steel) and corner key designs and dimensions. seal. The polyisobutylene PIB material itself was not modeled, due to the low strength of the material. Figure 4 shows the edge-seal configurations for the aluminum and galvanized steel spacers and the stainless steel spacer. FIG. 3 Stainless steel spacer and corner key designs and dimensions. FIG. 4 Edge-seal configurations for aluminum and galvanized steel and stainless steel spacers.

4 4 JOURNAL OF ASTM INTERNATIONAL FIG. 5 Corner deflection of aluminum spacer frame (exaggerated by factor of 100). Characterization of Material Properties For materials that display an almost linear stress-strain response within the given range of loads, such as glass, aluminum, and steel, the key physical properties required for FEA modeling Young s modulus of elasticity and Poisson s ratio are available in a number of engineering handbooks 5. The response of elastomeric seals and sealants, however, generally is nonlinear, even at lower strains. For silicone sealants, which by nature are closer in behavior to ideal elastomers, nonlinear response must be considered once strains exceed the range of about 15 % to +30 %. Linear FEA modeling becomes fairly inaccurate outside this limited range; therefore, a nonlinear stress-strain curve is required for the characterization of sealant behavior. Uniaxial tensile and compressive stress relaxation testing was used to determine the behavior of two silicone insulating glass sealants DOW CORNING 982 and DOW CORNING within the functional range of strain of the materials. These tests characterize the stress-strain behavior of the sealant at a specific temperature and after the sealant has had time to relax under strain. This approach is more representative of a quasistatic design condition where the applied sealant strains occur slowly, providing sufficient time for the sealant to relax and reach a uniform temperature 6. The tensile and compressive stress-strain curves are used with a curve fit program to create coefficients for a material constitutive equation. The constitutive equation provides strain energy density material functions for the elastomer portion of the model. FEA Calculations FEA is a numerical method for predicting the deformation of a part. Essentially, the part is broken down into a number of discrete entities or elements. A simultaneous analysis is performed on each individual element and the effect an element has on the neighboring elements. ABAQUS, a commercially available finite element analysis FEA software program, 5 was used for the modeling. FEA calculations were carried out to simulate the deformation of the edge seal for IGUs exposed to temperatures of 30 C 22 F and +60 C +140 F. IGUs with galvanized steel and aluminum spacers were modeled with DOW CORNING 982 as the secondary sealant; units with stainless steel spacer were modeled with DOW CORNING sealant. 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 corners are pushed outward, resulting in a bending angle less than 90. Figure 5 shows, as an example, the deformed corner 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- 4 Dow Corning Corporation, West Salzburg Road, Midland, MI ABAQUS Software, HKS Michigan, Sheldon Road, Suite 160, Plymouth, MI

5 STEWART ET AL. ON DIFFERENTIAL THERMAL MOVEMENT 5 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 1 Deformation of PIB primary seal as a function of spacer material. Deformation of PIB seal, %, for spacer material Temperature, C aluminum galvanized stainless steel +60 C +51% +42% +6% 30 C 60% 36% 4% FIG. 7 Nominal strain distribution in edge seal (aluminum spacer, DOW CORNING 982).

6 6 JOURNAL OF ASTM INTERNATIONAL FIG. 8 Nominal strain distribution in edge seal (galvanized steel spacer, DOW CORNING 982). 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 1. 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 ).

7 STEWART ET AL. ON DIFFERENTIAL THERMAL MOVEMENT 7 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. References 1 Wolf, A. T., Studies into the Life-Expectancy of Insulating Glass Units, Build. Environ., Vol. 27, No. 3, 1992, pp Feldmeier, F., Heinrich, R., Hepp, B., Schmid, J., and Stiell, W., The Aging Behavior of Insulating Glass (in German), Institute for Window Technology IFT, Rosenheim, Germany, Gravin, S. L., and Wilson, J., Environmental Conditions in Window Frames with Double-Glazed Units, Constr. Build. Mater., Vol. 12, 1998, pp Jacob, L., and D Cruz, J., Fundamental Concepts for the Design, Manufacture and Testing of IG Units for Warm Climate, Glass Processing Days June 1999, Tamglass Ltd. Oy, J. Vitkala, ed., Tampere, Finland, Chemical Engineers Handbook, 5th ed., R. H. Perry and C. H. Chilton, eds., McGraw-Hill Book Company, New York, Wolf, A. T., and Cleland-Host, H. L., Material Properties for Use in FEA Modeling: Sealant Behavior with Ambient Laboratory Climate Aging, Durability of Building and Construction Sealants and Adhesives, ASTM STP 1453, ASTM International, A. T. Wolf, ed., West Conshohocken, PA, 2004, pp