AGG

Volume 19,  Issue 1                                                   Spring 2005

The Tie That Binds
Adhesives and Sealants in Window Glazing Applications

As you scan the skyline of today’s cities, the effects of a little known hero can be seen everywhere. Modern buildings with massive expanses of glass reflect the blue sky and clouds above while offering a sense of openness in a once oppressive concrete jungle. It is the adhesives and sealants that are considered by some to be the unsung heroes of modern metropolitan architecture. Structural glazing attaches glass to a building under high wind and stress loads, yet the adhesives and sealants that strengthen and empower the structure remain unseen, and seldom recognized. 

Proven Success
One of the oldest adhesive technologies used in structural window glazing is silicone. Structural silicone glazing systems (SSGS) have a 30-plus year track record of success. Today the world’s tallest buildings use this technique based on the performance and durability demonstrated over the past decades. A key requirement for SSGS is the long-term adhesion durability to glass under the temperature extremes and movements found on the exterior of a commercial building. SSGS deliver a continuous rubber anchorage that allows an uninterrupted glass façade. The silicone allows for differential thermal expansion among substrates of differing thermal expansion coefficients. 

A variety of technologies are now used in window glazing and weather sealing, such as silicones, polyurethane reactive adhesives, acrylics, hot melt urethanes, synthetic rubber membrane systems and, more recently, tapes. These sealants bond glass to metal, stone, tile, ceramic, wood and composites.

For many, adhesives are the fastener of choice for glazing because they are aesthetically preferable if not invisible. Additionally, glazing adhesives provide sealing properties, low labor costs, high throughput rates, corrosion resistance and are lighter weight compared to old-fashioned mechanical fasteners.

Structural window glazing adhesives also provide manufacturing benefits including automation ability and compatibility with a variety of building materials.

Tested Performance
To assure reliable field performance, structural glazing adhesives must pass a rigid series of tests including UV-exposure, accelerated weathering, cyclic wind pressure-loading, structural performance, air infiltration, water resistance and performance under wide temperature and humidity variations. 

In terms of safety, glazing adhesives are flexible, forgiving and absorb shock better than mechanical fixation systems. Mechanical fasteners, which are rigid, cause localized stress points that can distort and weaken the glass and, in turn, the overall structure. Structural window glazing adhesives keep glass from contacting metal, and in the event of glass breakage due to vandalism, missile impact or severe seismic activity, the adhesive attachment can keep the shards of glass from falling down the façade. 

(Note: For SSGS the industry standard specification is ASTM C 1184 and the use guide is ASTM C 1401. These are both under the jurisdiction of ASTM Committee C-24 on Building Seals and Sealants. Current silicone technology demonstrates safety factors of 4-8 when designed according to ASTM C 1401 and tested according to ASTM C 1135. There are no other engineered, manufactured building components with such high safety factors.)

Forces of Nature 
Hurricanes and the associated risk of wind-blown debris highlight another powerful argument for the use of adhesives in structural glazing. Once an unprotected window has been compromised by flying debris the exterior pressures can severely load the structure, leading to lifted roofs, blown out windows and door walls, devastated commercial storefronts or even total collapse. 

As a result of the devastation of Hurricane Andrew in 1992, stricter codes for both commercial and residential buildings were developed such as the Metro Dade County protocols PA201-94 and PA203-94. These stringent standards of performance mandate higher levels of impact resistance for windows. The combination of structural glazing sealants and laminated window systems are proven to meet these new requirements, including withstanding winds up to 95 miles per hour, and can reduce significantly risks to the integrity of newly constructed buildings in hurricane prone areas. Designed for impact resistance, these systems provide outstanding adhesion, tear strength, flexibility and wind and moisture resistance. 

The use of adhesives and sealants are stretching the limits of architectural creativity. Adhesives allow for curved glass structures, breaking conventional block construction boundaries. The future looks bright for adhesives as architectural design trends continue to feature open, irregular spaces and natural light. 

This article was provided by the Adhesive Fastening Institute (AFI), which was developed by the Adhesive and Sealant Council’s Building the Industry program. AFI member companies Dow Corning, National Starch and Sika Corp. contributed to the development of this article: For more information on adhesives and sealants visit www.ascouncil.org or call 301/986-9700.


Architect's Guide to Glass & Metal
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