The advent of new elastomeric materials in the form of sealants, tapes and gaskets has introduced innovative, technically superior and aesthetically pleasing glazing concepts. Vijay Pillai, Deputy General Manager – Technical, 3M Industrial Adhesives & Tapes outlines the smart bonding technology for bonding the glass panel to structural frames and its other benefits in the fabrication process.
The frameless façade design of structural glazing is a delicate balance of aesthetic form and functional requirement. The load bearing structural frames are behind the glass, plane, and the glass is stuck to the frame with strong durable adhesive joints, most commonly with structural silicone sealants. The new generation joining system – VHB (Very High Bond) tapes – provides an ideal combination of performance, durability, and ease of use. In many situations, it can replace rivets, spot welds, liquid adhesives and permanent fastners.
The conventional method of bonding the structural glazing units to metallic frame is by using either single part or two part structural silicone adhesives. A double-sided open cell foam tape is used to give temporary holding of the glass and for creating the necessary space for the sealant. The dimension of the tape is decided based on the desired sealant bite and bead thickness. The bond should not be disturbed during the curing of silicone sealant, and it may take days to achieve the handling strength.
VHB tape provides an alternative bonding method with potential advantages that include faster assembly, lower total cost, immediate handling strength, improved aesthetics, low mess, consistent bond line thickness, reduced chance of read-through and greater design flexibility. VHB tape replaces both spacer tape and structural sealant and there is no curing reaction involved in bond strength development.
The first and foremost requirement of an adhesive selection for any joining application is the compatibility between the adhesive and the substrates involved. In glazing application, this ensures that the adhesive bond well with the glass surface and the aluminium structure. In addition, one needs to design the adhesive for sufficient cohesive strength and internal material strength so that it can withstand the various stresses and movement and act as an effective stress transfer mechanism. While designing the joint for glazing application, two modes of stresses due to dead & live loads acting on the glass panel are taken into consideration. The dead load is due to the weight of the glass panels used for glazing, which is acting vertically downwards, and sometime referred as vertical load or static load that causes shear stress on the joint. There will be live loads acting on the panel – mainly due to wind pressure and is also referred as horizontal load or dynamic load that causes tensile stresses on the panel. The adhesive joint also needs to accommodate various building movements due to seismic or differential thermal expansion contraction.
The generation of allowable or design stress values for the joining system requires knowledge of the application, the forces exerted on the bond and the time frame over which they could be expected to act. Tests are performed to arrive at design guidelines on allowable tensile and shear strength under both dynamic and static loads, in typical construction application such as bonding metallic frame to glass and other architectural panels in buildings.
Static & Dynamic load calculations: For a vertically-mounted glass panel attached to the outside of a commercial building, the most important forces to consider are static shear loads (due to the dead weight of the face material being supported by the VHB™ tape along the perimeter of the frame) and dynamic tensile loads (from a hurricane or other high wind event).
If the “ultimate” strength of the joining system is defined when complete failure of the adhesive bond occurs, the “design” strength value that structural engineers need to perform their calculations must contain some safety factor to be certain that failure is never reached. The engineer can then use these design values to calculate the area or the width of the tape required to hold a certain size of the panel against the specified wind load using the formula commonly followed by the construction industry – the most popular one is based on the trapezoidal rule. The wind load (dynamic or horizontal load) calculations are more important because in structural glazing designs the glazing unit is normally supported and the stress due to dead load is transferred to the main structure. The dead load support eliminates any long-term shear stress acting on the joint.
Joint Movement: In addition to the common short-term “dynamic” and long-term “static” forces discussed above, specific construction projects may have other types of loads that also need to be considered. The substrates involved in glazing in general are glass and aluminium, which have different co-efficients of thermal expansion. There may be diurnal forces that result from thermal expansion of these components during the daytime and result in forces on the adhesive bond that act for a time period of hours. The structural glazing VHB is a 2.3 mm thick tape, where as the minimum bead thickness recommended for sealant is 6.4 mm. The design shear strain for VHB is 300%, with a safety factor of 2 and it gives more than the design movement suggested for 6.4mm thick structural silicone bead.
Durability of the Joint: While the strength is the important design element of the structural glazing joint, its equally important to use a durable adhesive one that will not deteriorate its properties when exposed to weathering conditions for a long period of time. The ability of VHB tapes to withstand cold and hot temperatures, UV light exposure, humidity and other environmental conditions has been documented through both real-life and accelerated aging studies. The bond strength does not deteriorate below its original performance level, even after exposure of 10000 hours in most advanced Xenon Arc weather-o-meter tests.
Adhesion tests & quality check: It is critical that the tape have sufficient surface adhesion to the substrates involved to justify using the design parameters generated for the specific project under consideration. Evaluation of each application should thus include peel adhesion tests to the materials used in the project. Now-a-days glasses with different coatings and various coated metallic frames are used for glazing, making thorough compatibility studies necessary for each project, before giving recommendations. The adhesion tests also help to arrive at specific surface preparation guidelines for getting the necessary bonding strength. In structural silicone sealant glazing system, onsite quality checks are done to check for the curing of the sealant and air bubbles entrapment. Such deglazing checks are not practical and not necessary in the case VHB as there is no curing reaction.
Improving Aesthetics of Glass Façade: The use of pressure sensitive adhesive technology in place of chemically curing sealants offers many advantages to the glazing process. Structural adhesive tapes eliminate the non value added steps involved in the sealant glazing process thereby increasing the productivity. There is considerable saving in the process by avoiding accessory materials like spacer tape, masking tape, tooling etc. and the immediate bonding of the tape help faster turn around there by solving finished panel storage related issues. Most importantly the sharp bond line of VHB tape gives a much better appearance compared to sealant joints where the joint finish depend on operator skills.
Environment sensitivity: One very important industry trend has been the growing emphasis on sustainable or green building designs and indoor air quality is an important aspect in green designs. Use of materials with high VOC (volatile organic content) affects the indoor environmental quality very badly. VHB tape is a visco-elastic solid material and has no solvents/ monomers present in it. It has a VOC as low as less than 5 g/ litre which is much lower than the conventional Sealants (>50g/litre). Also it has excellent vibration damping properties that contributes noise reduction that may transmit through the bonding substrates.