Ways of Improving the Interfacial Durability of Silicone Adhesives in Building Applications

ABSTRACT: Beyond their application as sealants, silicone formulations are used as adhesives in insulated glass and structural glazing applications. The adhesion durability of such assemblies is a function of both the adhesion at the glass-interface surfaces and the internal cohesion of the adhesive. Fail­ure in the bulk of the adhesive will occur if external stresses exceed the ulti­mate strength of the adhesive or when the movement capability of the adhesive is lower than required by the application. The most often observed reasons for failure are, however, substrate-adhesive interface ruptures. If similar silicone adhesives are used in different building projects, the sub­strates can vary widely, leading to many different interfaces. Interface adhe­sion can be improved by modifying the substrate surface, modifying the adhesive formulation, or using a primer. In this work, improved adhesion du­rability on different substrates will be shown with an improved version of a two-part silicone adhesive used for insulated glass and structural glazing applications, which was introduced in Europe in 2010. Results without primer and with the use of either a wet primer or a dry SiOx (Pyrosil©) flame treat­ment will be shown. Adhesion durability after water immersion, UV irradiation, and high humidity will be reported for new low E insulated glass coatings, powder coated aluminum, stainless steel, and some other standard building substrates.

KEYWORDS: adhesion, durability, primers, silicones

Introduction

In the last two decades, mankind became increasingly aware of the scarcity of earth resources and the necessity to reduce their carbon footprint. Amongst the different sectors, the building sector is one of the most important carbon emit­ters and most of the energy is used to satisfy the heating demands of those build­ings. To reduce building energy consumption by more than 80 %, architects have improved insulation of walls and windows, reduced air leakage, and elimi­nate thermal bridges. Sealants and foams are widely used to improve the build­ing air tightness, whereas silicone adhesives used for structural glazing can be used to prevent thermal bridges due to their low thermal conductivity [1].

It has been demonstrated that very low-energy consumption buildings can be fabricated (sometimes also called passive houses), however, people need to ensure that excellent air tightness and insulation is maintained during the life­time of the building, which is currently expected to be at least 50 years, but of­ten up to 100 years.

In the building industry, silicones are used as sealants to air-tighten build­ings and as adhesives to structurally bond glass to facades (Fig. 1) or maintain the insulated glass integrity (Fig. 2). Those sealants or adhesives are subject to numerous different environmental stresses. When bonding two substrates with different coefficients of thermal expansions, thermal elongations are induced by temperature changes between day and night or summer and winter. Constant

FIG. 1—Skyscraper – structural glazing with insulating glass.

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FIG. 2—Structural glazing application.

deformation in a sealant can also occur due to earth movements or building set­tlement. Furthermore, when the sealant is applied outdoors, those stresses are combined with exposure to rain, ozone, and sunlight. Due to their high Si-O energy bond, silicone sealants are especially suited for outdoor applications (Fig. 3). The durability of a sealant/adhesive is a function of both the adhesion at the sealant-substrate interfaces and the internal cohesion of the sealant. Fail­ure in the bulk of the sealant will occur if external stresses exceed the ultimate strength of the adhesive or when movement capability of the adhesive is lower than required by the application. It has already been shown [2] that, due to their excellent elastic recovery, silicone sealants outperform organic sealants when subject to combined cyclic movements and exposure to UV/humidity.

The most often observed reasons for failure are, however, substrate-sealant interface ruptures. If similar silicone sealants or adhesives are used in different building projects, the substrates can vary widely, leading to many different interfaces. When adhesion does not occur on certain substrates the adhesi – ve/sealant formulation needs to be improved, the substrate surface needs to be modified (using plasma treatment, for instance), or primers can be used.

In this work, adhesion durability on different substrates used in glazing applications is evaluated, comparing 2K silicone adhesives used for insulated glass (DC 3362) and structural glazing applications (DC 993) with a formulation which was optimized (DC opt) for adhesion build-up. The effect of the applica­tion of a wet primer (1200 OS Primer Clear, Dow Corning [3]) or a dry primer

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(Pyrosil®, Bohle AG [4]) was also evaluated. This study concentrated on adhe­sion on low E glasses (soft coatings on face 2) which is a key contributor for improved glass energy efficiency. Different substrates used for the structural bonding of glass in glass facades or in solar thermal panel applications were also evaluated.

The wet primer was applied with a brush as a thin film. The dry primer (Pyrosil®R) was applied from a small hand burner (Fig. 4), which applies a thin

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SiOx layer on the surface, which is obtained by atmospheric pressure combus­tion chemical vapor deposition of a silane/gas mixture [5].