# Primary Seal Deformation in Multipane Glazing Units
## 1 Introduction
### Issues and Limitations of IGU Durability  
The major limitation regarding the service life of an IGU is vapour condensation fogging in the insulating chambers, i.e., on the internal glass surfaces. The durability of units is therefore related to the water vapour permeability of the edge seal, which was experimentally examined in [5], where time to fogging was predicted based on measurements of the dew point temperature of the cavity gas fill in IGUs under several ambient conditions. The moisture content in the insulating chambers was experimentally examined in [6] and can be assessed using standards for IGU durability testing [7]. A novel experimental method for diffusion gas loss in IGUs was presented in [8]. Multipane glazing units are attractive due to high energy efficiency of those buildings where they are built-in [9]. However, in multipane glazing arrangements (i.e., in glazing with more than three glass panes) a further restriction stems from the maximum permitted operating temperature of the PIB primary sealant. In two-pane and three-pane arrangements an 80 \({}^{\circ}\)C temperature limit [10] is difficult to exceed; however, exceeding the maximum permitted PIB temperature is easy in multipane arrangements [10]. The water vapour permeability of the dual-edge seal is almost exclusively determined by the permeability of the primary sealant because the permeability of the secondary sealant is significantly lower [11]. However, the diffusion process, as a main component of the gas permeability in the primary sealant, is exponentially dependent on temperature [12], hence resistance to atmospheric loads is the lowest during the summer. Not only a change in the material properties, but also the deformation caused by climatic loads plays a crucial role in the longevity of any IGU. Increased temperature causes a pressure increase in the insulating chambers. The pressure deflects the glass panes and the consequent change in the pressure and chamber volume is dictated by the ideal gas law. Thin glass panes enable a higher chamber volume change due to lower stiffness, which compensates for the pressure increase and lowers the pressure load on the edge seal. However, high edge seal displacements are not acceptable from a durability point of view and also due to the potential risk of glass failure [13]. Therefore, glass panes normally feature significant stiffness, which results in a pressure increase in the insulating chambers that further stretches the edge seal. Such stretching has a direct influence on moisture permeation because the deformed effective diffusion cross-section of the primary seal increases and the diffusion length decreases. A double-seal system between two glass panes is made of two layers perpendicular to the glass: in the first one there is a spacer and a primary sealant, and the second layer enveloping the first layer represents a secondary seal. From a mechanical point of view, there are two parallel elements subjected to the same displacement, but the strains in the two sealants differ significantly. While the strain in the secondary seal is nearly homogeneous, the primary seal has to compensate for the vast majority of the displacement due to the underlying spacer stiffness. The described phenomenon is experimentally well documented together with the development of a new type of spacer in [14] and confirmed by measurements of the gas concentration decrease in the IGU's insulating chamber [15]. Therefore, in a typical IGU, the strain on the primary sealant is significantly higher than the corresponding strain on the secondary sealant. Furthermore, the material properties of both sealants also differ significantly. The primary sealant is a thermoplastic-like liquid and is not employed as a structural component. It stretches and compresses as it is forced by the surrounding structural components. As long as the elastic deformation remains small, cyclic straining is mainly reversible.