Factors Influencing the Durability of Sealed Joints and Adhesive Fixing

While our understanding of the factors that determine the service life of sealed or bonded joints has progressed substantially over the past decades, there is still much research needed on the durability and reliability of novel structures, components or designs. Several papers at the symposium focus on this topic.

Bent or warped glass allows turning a typical glass-and-metal curtain wall design into an exciting, innovative architectural statement. Traditionally, curved glass is manufactured from float glass by heating it to a temperature above the softening point and then shaping the glass in a mould. Since this

technique is time and energy consuming and consequently relatively expen­sive, cold-bending has been developed as a more affordable alternative. In this glazing technique, flat glass panes are bent to the desired shape on a curved frame and then mechanically or adhesively attached to the frame. The cold-bending process implies that the glass becomes permanently subjected to bending stresses during its lifetime. Glass on contemporary curtain wall projects is mostly insulating glass which raises concerns about the longevity of cold bent insulating glass (IG) units, as the bending process induces a shear­ing action to both the primary and secondary edge-seals. While very little sci­entific information on this topic has been generated in the past, the number of building projects involving cold-bent insulating glass globally continues to increase rapidly. In their land-mark paper, Besserud, Bergers, Black, Car­bary, Mazurek, Misson and Rubis describe testing protocols designated to determine the effect of cold-bending on the durability of the insulating glass unit as measured by argon retention, frost point change, and visual changes after aging. As part of the experimental protocol, first the bending behavior of a full size IG unit is assessed, which is then modeled to predict the stresses and strains on the primary and secondary seals. Small (standard) size IG units are then tested according to the ASTM E2188-10 and E2190-10 protocols while simultaneously subjecting them to an edge seal displacement in all three di­rections that induces equivalent stresses in the edge seal. Argon retention and frost point measurements are taken before and after the durability testing and results reported. The methodology developed in this research provides a strong foundation for future testing in the area of cold-bent IG unit durability.

Durable, reliable, and high strength adhesion of elastomeric sealants and adhesives to a variety of substrates is essential to a broad range of industries. In their paper, Gutowski, Toikka and Li discuss and experimentally verify the principles of engineering substrate surfaces through grafted connector molecules. The authors demonstrate that the incorporation of silicon-based and/or amine-terminated graft molecules such as silanes or polyethylene – imines, at the polymer interface, results in the formation of strong molecular links between a range of organic and metallic substrates and elastomeric sealants or adhesives, leading to significantly improved bonding. The tech­nology has been successfully adopted by the global automotive industry for improving adhesion of a variety of adhesives and coatings to polyolefinic substrates.

The bonding of point-fixed supports for glazing has recently received in­creased attention, as in contrast to mechanical fixation, bonding of point supports offers a number of advantages, such as no or lower visibility of the supports from the exterior, a ‘smooth’ transfer of the load into the glass pane (avoiding stress peaks), and the elimination of drilling holes from the glass. Failure mechanisms under typical loading conditions and parameters that

affect failure probability, mode, or mechanism are the focus of ongoing inves­tigations. Hagl studies the mechanical characteristics of degraded silicone – bonded point supports with axial geometry undergoing tensile loading. Ten­sile loading of bonded point supports is considered the critical load case, as dynamic loads, such as wind load, subject the adhesive to out-of-plane loads. In the paper, the following parameters are investigated in their effect on the durability of point supports bonded with a two-part adhesive: (a) incorrect mixing ratio of the adhesive components, (b) inhomogeneous mixing due to insufficient or improper mixing procedure, (c) fatigue degradation of the ad­hesive, and (d) local defects in the bond, e. g., caused by inclusion of bubbles or by partially failed adhesion. The main motivation for this kind of research is to strengthen the confidence of building code authorities in the durability of bonded designs.

As mentioned earlier, there is an increased interest in the cold-bending of glass in order to realize curved or warped glass fagades. However, cold bend­ing induces permanent stresses in the glazing structure, especially in the corner area of the glass units. Dynamic or static loads acting perpendicular to the glass surface, such as wind or snow loads, also cause high stresses in the corner area. Hagl and Dieterich present numerical results of a para­metric study for pressure-loaded glass units with a focus on corner loads and stresses. The results show that the stress levels in the corner areas might exceed the design stress values used for sizing the bond geometry.

Blistering of sealed or bonded joints is as a form of degradation. Some­times blistering is observed when exposure to direct sunlight occurs imme­diately after application of the sealant or adhesive on an unusually hot day. Often this case of blistering can be attributed to intrusion of air or moisture from voids within the substrate into the sealant or adhesive. While other causes of blistering exist, blistering driven by the diurnal variation in tem­perature is an important aspect of the degradation of sealed or bonded joints. The paper by Hailesilassie and Partl deals with the mechanism of asphalt blistering on concrete bridges. While the focus of their paper is on blistering in asphalt overlays, their findings are relevant to the sealant and adhesive industry. According to the authors, blistering is a major problem in asphalt covered concrete structures, such as multi storage parking buildings, built – up roofs, tunnels, pedestrian areas or concrete bridge decks. In this particular research, a linear viscoelastic finite element model is developed to simulate time dependent blister growth in the asphalt layer under uniformly applied pressure with and without temperature and pressure fluctuation. The finite element model simulation shows that the daily temperature variations may have a significant influence on blister growth in asphalt pavements. The authors conclude that temperature fluctuation has more influence on blister growth than fluctuation of the pressure inside the blister.

Joints may fail because of degradation of either cohesive or adhesive prop­erties of the sealant or adhesive. Since silicone materials display excellent bulk durability, adhesive failure mode is the more likely cause of joint failure. Interfacial adhesion can be improved either by modifying the formulation of the sealant or adhesive or by modifying or treating the surface of the sub­strate, for instance, by plasma treatment or use of a primer. Vandereecken and Maton report on a comparative study evaluating the adhesion improve­ment observed for a two-part silicone adhesive on a variety of substrates either by applying a wet primer or the dry Pyrosil® flame treatment. Pyro – sil® is a pyrolytic chemical pre-treatment process that forms an amorphous, nano-scale silicate layer on the treated surfaces. In this process, the targeted surface is treated with the front (oxidizing) section of a flame obtained by burning a silane, propane, and butane mixture in a pen-like torch. During the combustion process, the silane is oxidized to form SiO2 nano-particles which cover the surface with an ultra-thin (20 – 40 nm) strongly adhering silica coating.