Reducing Modulus of Sealants to Improve Durability
ABSTRACT: The premise is that if one reduces sealant fatigue by lowering modulus while leaving other characteristics of a sealant unchanged, its durability will improve. Reducing sealant fatigue is seen as a positive consequence of lowering the modulus of elasticity of a sealant. The data shows the effect of incorporating a modulus modifier in one – and two-part (standard curing) urethane sealants and a one-part silane curing polyether sealant. This approach lowers modulus with no or minimum reformulation.
KEYWORDS: sealant, additive, adhesion, modulus, elongation, movement, durability Introduction
Most sealants are not used as structural adhesives. The majority of sealants are intended to fill holes and joints and move when the joint moves and adhere the whole time. Sealants that are somewhat softer but remain elastomeric are more suited for those applications having high joint movement like expansion joints, curtain walls, highway pavement joints, parking decks, and bridges. For example, a sealant that can take +/-25 % joint movement is considered more useful and can be used in more applications than a sealant +/-12.5 % or +/-20 % joint movement. That is true right up the line, with +/-30, 40, and 50 % joint movement all being more useful. Therefore, additives that lower the modulus of elasticity can be very useful if indeed they lower the modulus of elasticity and are accompanied by higher elongation. The key benefit seen is that they are more capable of handling higher joint movement while lessening stresses on the substrates resulting in greater durability and longevity. This is especially important if the substrate is relatively weak like Exterior Insulation Finished System (EIFS) or with many cementatious surfaces. In essence, the joint lasts longer when less stress is introduced. When joints last longer, buildings last longer.
To that end, a melamine type additive was evaluated in different concentrations and solvents with several sealant types to see if a beneficial change in performance could be achieved.