Durability of Pressure Sensitive Adhesive Joints

Pressure sensitive mounting tapes provide a quick and effective fastening method compared to most mechanical joining techniques. Due to the viscoelastic nature of pressure sensitive adhesives their response to mechanical and climatic stress in terms of durability is different from most structural adhesives. While the feature of compensating differences in thermal expansion of bonded members and the ability to recover during cyclic loading add to their superior properties, in specific applications their characteristic creep behavior can be detrimental in other areas. To obtain design values for adhesively bonded joints with pressure sensitive tapes, specific testing methods are required to access their longterm properties under environmental exposure. In this study standard test procedures for structural adhesives including static shear and cleavage tests have been adapted for semi structural bonds with adhesive tapes and applied to metal and glass joints under standard aging conditions. The results reveal future opportunities and limitations in using pressure sensitive tapes in demanding applications. Introduction Adhesive bonding has become a preferred fastening technology in applications where dissimilar materials need to be joined. In glazing applications pressure sensitive tapes have been successfully used to bond glass panels to supporting metal framework (Fig. 1). Here the adhesive fixture avoids stress concentrations in the crack-sensitive glass panel and compensates the difference in thermal expansion between glass and aluminum. Figure 1. Bonding of laminated safety-glass panels to aluminum framework with pressure sensitive tape Fail-safe design concepts of load bearing joints with pressure sensitive adhesives need to consider their specific mechanical long-term behavior which relates to the viscoelastic material properties of pressure sensitive polymers and blends. Prior to approval several durability tests need to be carried out to investigate and prove the durability of a specific substrates-adhesive combination against the following failure mechanisms: • excessive long-term shear creep deformation under static and cyclic load conditions • local cohesive failure due to crack formation and propagation in the adhesive bulk • interfacial loss of adhesion upon exposure to damaging weather conditions Failure Under Shear Stress In a typical glazing application static shear deformation will occur when the adhesive layer is designed to carry the dead load of a glass panel. Additional cyclic shear deformations are usually superimposed due to the difference of thermal expansion between the glass panel and the framework. The resulting maximum stress and strain level of the adhesive tape in such situations depends on the overall length of the bonded members and the thickness of the adhesive tape. The resulting normalized shear strain tan γ can be calculated through division of the total displacement ΔL by the bond line thickness d as indicated in Fig. 2. glass panel aluminum frame