A Crucial Component of the Glazing Equation

Past and Future Breakthroughs from Structural Sealants

Editor’s note: This is an expanded version, with additional content, of the feature that appears in the August issue of USGlass on page 66—our special anniversary issue.

The commercialization of the float glass process in the 1960s had designers and glazing contractors alike thinking that glass was ready for bigger things. But glass is only one part of the equation when it comes to glazing. Within just a few years, silicone sealants emerged as a solution for bonding glass to metal, and since that time sealants have played a crucial role in adding to glass’s capabilities—and their manufacturers are still exploring new ways to provide added strength, and value, to complex glass creations.

 Structural Silicone Glazing Technology: Today’s Benefits and Tomorrow’s Possibilities

By Lawrence D. Carbary and Jon H. Kimberlain

Structural silicone glazing (SSG) is a technique of attaching glass to a metal frame using structural silicone adhesive sealants. This method was welcomed initially due to the architectural design freedom it enabled for commercial building structures. Its widespread use is showcased globally in curtainwalls of the commercial buildings in city centers. This technique of attaching glass using a continuous anchor of flexible silicone rubber has become even more widely accepted due to its superior long-term performance with regards to resisting air and water infiltration, earthquakes, UV, acid rain, extreme weather events, and impact loads—all while maintaining thermal performance and the aesthetics of a smooth glass façade.

Continuous application of structural silicone along the perimeter of the glazing has some unique performance attributes. This method prevents shards of laminated glass from falling from the opening in the event of glass breakage, whether caused by natural or unnatural sources. The system’s proven long-term performance benefits building owners by keeping commercial properties leak-free, energy-efficient and aesthetically pleasing.

Because of its proven performance, the system continues to gain in popularity and use. The basis for structural silicone design strength and method of design has been used in projects around the world and has performed for more than 40 years. ASTM C1401 Standard Guide for Structural Sealant Glazing, the most complete reference to the design considerations for structural silicone glazing, provides an excellent overview of the practice along with a full list of historical references.

Factory application has become the preferred method of structural silicone glazing and panel attachment within the curtainwall industry. This is because the factory environment can be environmentally controlled, quality control practices are easier to implement, and supervision is readily available compared to the field environment.

With the installation process of unitized curtainwalls an accepted practice, the expectation of fast wall erection continues based on current performance of the curtainwall installers. The architectural statement of the structure – the curtainwall – is installed relatively quickly, giving the building its architectural appearance many months before building tenants are able to move in.

Tomorrow’s Realities

Requirements for curtainwall in today’s building codes have been influenced by recent extreme wind events. Design wind speeds and cladding pressures specified in ASCE7-10 have increased significantly, resulting in correspondingly larger material sizes and strength to achieve sufficient structural capacity. For example, tall buildings in hurricane zones have cladding pressures that now routinely exceed 120 psf (6 kPa). These very high cladding wind pressure magnitudes result in bulky SSG profile dimensions.

Some efforts to challenge both the basis of designing structural joints and increasing design strengths have been proposed to reduce the use of aluminum – which can impact the thermal performance and overall cost of the curtainwall project – and to increase vision area and improve sightlines for better aesthetics. Unfortunately, there have been no thoroughly developed and published technical arguments for proof of challenges to designing structural sealant joints and increasing design strengths; these appear to be opportunistic approaches to increase design strength based on business risk and reward rather than sound science and engineering.

To meet the architectural desire for aesthetically slender curtainwall framing sight lines, SSG designers have been prompted to find a way to optimize the silicone joint.

During the optimization process, two advances in structural design and engineering have been proposed by the design and engineering community. First, nonlinear analysis of hyper-elastic material is used to replace the industry standard assumptions, which use rigid supports, uniform load distribution, small deformations and linear material behavior. Second, reconfiguring the silicone joint geometry from rectangular to trapezoidal shapes provides improved stress distribution and allows the design of a more efficient structural connection.

Advanced science and engineering will optimize the design of structural silicone glazing while ensuring continued proven performance for building owners. This design optimization must take place in conjunction with curtainwall installers to avoid obstacles that would require unnecessary additions to manufacturing and installation processes.

Tomorrow’s Possibilities

Optimized structural silicone joints will result in smaller framing systems that safely support glazing with higher specified loads and more energy-efficient specified wall designs. The performance of structural silicone will move beyond just glass and metal to other façade substrates.

Higher-strength, heat-cured silicone materials will be accepted with higher design-load ratings to be used as point adhesives, further reducing overall contact and changing the aesthetics of glazing designs. Use of advanced science and engineering to create hyper-elastic models, which can be tested in a mock-up to prove the concept, will bring radical aesthetic changes to façade and building design.

Lawrence D. Carbary, Construction Industry Scientist, the senior member of Dow Corning’s Construction Technical Service staff, works on new technologies for commercial façade insulation, sealing and glazing techniques.

Jon H. Kimberlain, application specialist for Dow Corning, provides expertise in the use of silicone sealants for the building profession in markets including fenestration, commercial building projects, structural glazing and pavement. 

Sealant Roundtable

Structural Silicones Step Up

Evolving architectural designs have placed increasing demands on glazing materials, including silicone sealants used to support the glass. In this roundtable discussion, Tremco glazing experts Bill Cardott and Dave Horshig of the building envelope solutions team and glazing specialists Jeff McGovern, Joe Dressler and Jeffrey Donlon expound on the past and the future states in structural glazing.

History of Structural Silicone Sealants

Cardott: One critical early development was driven by architectural design. Early high-rise construction featured anodized aluminum window systems, but perimeter joints had opposing interfaces of mineral composition. For the earliest silicones, acetic acid was a byproduct of cure. While not a problem for metal-to-metal, metal-to-glass and glass-to-glass interfaces, it was not compatible with the mineral substrates at window perimeters. This spurred development of silicone sealants that exhibited neutral effect on those substrates.

The first neutral-cure silicone sealants exhibited other desirable characteristics as well — most notably, very high movement capabilities and low-modulus. The high-movement capability makes sense since perimeter joints typically have opposing joint interfaces of different composition that exhibit differing rates of expansion and contraction. Movement in window system perimeter joints is a given. Early neutral-cure silicone sealants greatly exceeded the ASTM C920 Class 25 for ±25 percent joint movement capability — the highest recognized movement class at the time. They also offered the simplest solution to undersized joints that resulted from inadequate consideration of erection tolerances of opposing substrate components.

The low-modulus characteristic also is significant because the stress of joint movement is largely absorbed within the sealant body rather than being channeled to the bond line. This helps avoid sealant joint failure at the bond line and failure of the substrate.

Horshig: Neutrality of silicone sealant cure has become desirable for other reasons as well. Acetic silicone sealants used in glazing caused the silicone secondary seals of structural IG units (introduced to save energy) to fail. So it was critical to the industry to introduce, validate and utilize neutral cure silicones.

Cardott: Safety has been a driver in the evolution of structural silicones. While the earliest silicone sealants exhibited a tensile strength of 100 psi, factored into the tensile-bead sizing formulae still used today, they have evolved to exhibit higher tensile strengths — critical in seismic and hurricane zones. While the movement capability of some two-component silicone sealants was ±12 ½ percent, evolution has resulted in ±25 percent movement sealants gaining preference. The ±25 percent movement two-component structural silicones perform throughout pulse-cycling during hurricane-impact testing. In a collaboration of three silicone sealant manufacturers, testing evaluated fatigue properties, and the results showed that after 36,500 cycles of the simulated joints, sealants with higher movement capability showed less susceptibility to degradation than lower movement sealants when strained to 15 percent. In other testing, four-sided structural silicone glazing was subjected to seismic movements and proved desired performance, leading to adoption in the 2013 California Building Code, Section 2410.

Present and Future of Structural Silicones in Glazing

McGovern: While decades ago Hurricane Andrew prompted code changes including strong hurricane provisions for glazing systems, in more recent years, states in the southeastern United States have finally started modifying codes for energy efficiency.

To meet increasingly stringent energy codes in Florida, curtainwall fabricators are moving toward the use of insulating, laminated glass. Unitizing is also becoming more common to speed production and ensure quality and energy performance. Hurricane-rated systems, in particular, rely heavily on the high-quality bonding of structural silicone.

As energy efficiency ratings become critical, building owners and consultants turn toward single-source suppliers to guarantee continuity of the entire building facade — not just the glazing system but all the critical components, such as transitions and air/vapor barriers.

Dressler: Building facades are becoming more complex, and the materials utilized on the opaque wall requiring glazing system tie-ins vary greatly, including fluid membranes, sheet goods, synthetic, asphaltic and foil facings. For a securely sealed building envelope, the glazing sealant must be compatible with all of these.

Managing this challenge is best done by pre-construction due diligence. During the pre-con meetings, submittals for all trades should be reviewed and coordinated. In-place mock-ups should be done to confirm the substrates involved and show true project representations. Documentation should be provided from manufacturers to ensure that proper adhesion and compatibility testing has been done.

McGovern: Building designers’ move toward larger and heavier lites of glass has increased design pressures as well. There is a direct correlation to mullion sizes increasing to meet the applicable loads and to include more structural silicone to the bond line. These changes are pushing sealants to designed limits.

Donlon: Sealant manufacturers and designers now need to be included in the process earlier to have insight into the designed intent, to be able to meet those needs more efficiently, and to formulate new products if necessary.

Additionally, the software employed by higher-end fabricators, installers and designers typically does not take into consideration how many parts are held in place or installed. Sealant manufacturers need to be able to assist them in the practical aspects of constructing these next-generation façade designs.

The Evolution of Glazing Sealants

By Carl Tompkins

Sealant products have been around a long time, going back to as early as the 17th century when various types of man-made putty were used to seal window glass installations. Linseed oil and chalk were the main ingredients then. As we moved ahead into the early 1900s, the development of sealants took on a much more sophisticated design. Simple oil- and resin-based caulks were introduced in the 1950s, which were reasonable in weather-sealing performance but provided little to no elasticity that resulted in little joint movement capability. Polysulfides were next to arrive on the scene in the early 1960s; they provided an increased joint movement capability, being the first truly elastic sealant.

A Sealant for Construction

The first true construction sealant, developed by GE, followed in the form of a silicone product that provided increased tensile strength, along with a joint movement capability of plus or minus 25 percent. This innovation in sealant technology allowed for flexibility in the design and size for window and door systems. The two downsides to the introduction of silicone were its acetoxy cure mechanism (high odor release and possible staining of substrates) and its inability to be painted once fully cured. Silicones continued to evolve during the decade, arriving to the introduction of neutral cured products having even greater elastic qualities than their earlier rivals. The development of two-part systems added faster curing products that delivered greater strengths for structural sealant glazing.

The 1970s saw the introduction of polyurethane technology that provided the same joint movement capabilities along with the ability to be painted. That provided greater abrasion resistance than silicones. To this day, polyurethane and silicones continue to dominate as the two categories of commercial glazing sealant types.

Acrylic- and butyl-based products have come along in this journey of sealant developments, with the noted benefit of being the most economical choice. To date, however, they have not been able to meet the full list of performance requirements for commercial weather sealing.

The more recent trend in sealant development comes in the form of MS polymer and hybrid systems, where sealant types are blended into a combined formulation in an attempt to provide the best of product features: a paintable surface requiring little surface preparation of substrates that is UV-resistant, provides high joint movement capability and offers a wide temperature range of performance. Currently, the majority of this technology is used in the residential sector.

Finding the Right Function

As sealants have evolved, there has remained one key function of weather sealing, which is the importance of selecting the right sealant for the job at hand. My own most significant advice to glass companies and their glaziers is that while sealants represent one of the smallest costs in the scheme of a commercial or large residential glazing project, they represent one of the highest risks of expensive rework.

Essentially, the purpose of a sealant is to provide a functional seal between two or more sides of a joint to prevent moisture, dust, light or air from penetrating into or out of a structure. As it pertains to the flat glass industry, the fenestration process requires weather sealing not only to make the installation of glazing systems airtight, but also to provide a lasting, cosmetically appealing appearance.

In summarizing the more recent feature improvements of commercial glazing sealants, they are faster in cure, provide greater strength and flexibility, and require fewer steps in their proper application. This adds up to providing glazing contractors improved productivity and reduced risk in rework. For architects and developers, greater flexibility in building design often relates to larger window systems done in more non-traditional ways, providing greater cosmetic appeal. For the owners of buildings, an improved finished appearance and performance. Again, be sure to pick the right sealant for the job and follow the manufacturer’s usage instructions for getting the right job done the first time.

Carl Tompkins is national sales manager of flat glass for Sika Corp.

Special thanks to Rick Mathis, flat glass program manager with Sika, for his assistance in the development of this article.

 

1 Comment

  1. For SSG systems it’s very important for the structural sealant bite to be engineered so that there is enough adhesion surface.

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