Preliminary Evaluation of the Mechanical Properties and Durability of Transparent Structural Silicone Adhesive (TSSA) for Point Fixing in Glazing

ABSTRACT: The paper reports on the preliminary evaluation of a transparent structural silicone adhesive (TSSA) developed for point fixing in glazing, which combines high transparency, strong adhesion performance, thermal stability, and excellent weatherability. The transparent film adhesive is a heat curing one – part material that shows strong bonding to glass, metals, ceramics, and even plastics typically without primer. The paper presents information on the durability and physical properties of the new material and suggests a methodology for deriving static and dynamic design strength values for the new material based on creep rupture experiments as well as nondestructive dynamic load experi­ments using the stress whitening phenomenon observed with this material as the limit state. The paper further discusses material characterization and hypere­lastic modeling used in the finite element analysis based on finite strain theory.

KEYWORDS: structural, silicone, film adhesive, point fixing, glazing Introduction

Glass is widely used in contemporary architecture as transparent infill elements in the building envelope because of its aesthetic characteristics. The use of glass

also permits passage of light into the building, which is important for the well being of the building’s occupants. The glass panes may be fixed to the support­ing structure by either linear or point bearings. The linear supports attach the glass pane to the substructure on two, three, or four sides. In this system, the glass pane may either be retained on both faces in a glazing channel by a flexible gasket made from EPDM, chloroprene, silicone, or a similar material, or adhe­sively attached on one face by a structural glazing sealant. Point bearings can be classified into fixing clamps and point-fixed supports. The point-fixed sup­ports are generally positioned in the vicinity of the corners of the glass pane and retain the glass pane either mechanically (metal bolts penetrating the glass) or adhesively [1,2]. Mechanical fixing of glass panes furnished with holes requires the designer to pay attention to the placement of the holes in order to meet the requirements of national standards. For instance, ASTM C 1048 Specification for Heat-Treated Flat Glass [3] specifies that the hole must be placed at a dis­tance of at least 6.5 times the thickness of glass away from the corner. Further­more, in order to deal with unavoidable stress concentrations around the fixing holes, heat strengthened or tempered glass must be used.

Bonded point-fixed supports have recently received increased attention, as in contrast to mechanical point supports they offer a number of advantages, such as no or less visibility from the exterior, a “smooth" transfer of the load into the glass pane (avoiding stress peaks), and the elimination of drilling holes into the glass [4-7]. Contrary to the glass panes, the adhesive fixing used in either linear or point bearings may experience both out-of-plane and in-plane loads, depending on whether the dead load of the glazing element is carried by mechanical setting blocks into the building envelope substructure. Based on the current state-of-the-art, the adhesive fixing of glazing elements in exterior (building envelope) applications is limited to structural silicone sealants and, more recently, to acrylic pressure-sensitive adhesive (PSA) coated structural foam tapes [8].

Structural silicone sealants have been used in linear adhesive fixing of glaz­ing elements at a tertiary structural level since the 1960s [9]. The long-term ex­perience with silicone sealants in this field has led to standardization of both the performance requirements on the structural adhesive sealants as well as the glazing designs [10,11]. The room-temperature-vulcanizing (RTV) structural silicone sealants used in linear fixing of glazing elements (structural silicone glazing) display a low Young’s modulus (generally in the range of about 1.0-2.5 MPa in tension) and a high elongation at break (generally in the range of >100% when measured in tension on a tensile-adhesion joint with dimen­sions as defined in ISO 8339 [12]). The resulting joint design allows compensa­tion of thermally induced movements and dimensional tolerances between the substrates, which is a necessity for linear structural bonded bearings. However, for adhesively bonded point-fixed bearings, a higher Young’s modulus is desira­ble to achieve higher stiffness with a smaller bonding area that still allows carry­ing significant out-of-plane loads [6].

Recent studies have focused on the evaluation of photocured acrylics for this application; however, these materials still suffer from limitations, such as insufficient water resistance and application issues (watery thin viscosity),

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which currently restrict their broader use in exterior (building envelope) appli­cations [4,5]. Therefore, it would be highly desirable having a transparent sili­cone material available that combines the inherent durability of the siloxane polymers [13] with an improved strength, suitable for adhesive point fixing, and a simple application method.

This paper presents experimental data obtained on a one-part, heat-trig­gered addition-cure structural silicone film adhesive. It also discusses proposals for the development of design strength values for the material when exposed to dynamic or static loads.