Screw slots
3.13 STRUCTURAL SILICONE GLAZING COMPONENTS
S
tructural silicone glazing (SSG) is a specific glazing method where the glass is adhesively held in the framing, on either two or four edges, with a structural grade silicone sealant. The method is properly part of a glazing system but many of the components are unique to the system or require more attention than batten or stop glazing.3.13.1 Framing
All of the comments regarding curtain wall framing including alloys, corrosion potential, and galvanic reaction, mentioned in Section 3.3, apply to framing for structural glazing systems. Differences lie in the method of retaining the glass. Stick systems field glazed (two-sided SSG) should be designed for application of sealant from the interior as shown in Figure 14. The finished silicone bead forms the interior gasket. The frame section must present a smooth flat surface for the silicone to bond to along with some means of clamping the glass temporarily until the silicone cures. The cure time varies with sealant and weather conditions. Manufacturer standards recommend 21 days of curing to achieve full strength. Removal of temporary clips prior to 21 days can be done with approval of the sealant manufacturer.
In unitized systems, incorporating either two- or four-sided SSG, the silicone is applied from the outside as there is clear access before the mating sections are joined (Figure 24). In both stick and unitized systems the mullion head must have provisions for either gasket or foam tape application.
3.13.2 Gaskets and Tapes
The primary purpose of gaskets or glazing tapes in an SSG system is that of a spacer to hold the glass at a set distance from the framing. This controls the thickness and, along with the gasket/tape position, the profile of the structural silicone sealant bead. The gasket or tape may also function as a finished edge as in the case of an interior gasket.
The most critical characteristic of the glazing gasket or tape in an SSG system is its chemical compatibility with the structural silicone sealant and the silicone weather seal. Given the importance of the long-term integrity of the structural silicone bead no chemical incompatibility is acceptable. The major silicone manufacturers have developed a series of standard tests to evaluate the potential for chemical incompatibility between the silicone and any contacting material, in this case the gasket or tape. These tests have been formalized into ASTM C-1087-95. The tests are conducted on production run materials and take approximately four weeks to complete.
The most common material for extruded glazing gaskets on SSG systems is silicone. While verification is required, silicone gaskets are of the same basic material as the structural silicone bead and this results in a low-risk of incompatibility. The most common glazing tape is a dense polyurethane foam manufactured with adhesive on one or both sides. The adhesive is used to temporarily hold the tape in position until the glass is installed and the silicone sealant cured. Unlike the silicone gasket, polyurethane is from a different chemical family and only certain formulations provide the necessary
compatibility. The major silicone manufacturers prequalify specific polyurethane tapes made by specific manufacturers. Even though some tapes are prequalified, testing of all tapes should still be conducted to verify compatibility.
3.13.3 Glazing Accessories
As with the gaskets in an SSG system, chemical compatibility of all glazing accessories that might contact the silicone sealant is a prime requirement. The setting block and backers to the silicone weather seal are the main elements of concern.
The setting blocks must possess the same qualities as outlined in Section 3.6 in addition to chemical compatibility with the structural silicone. This leads to the use of either silicone or silicone-compatible rubber setting blocks. Compatibility testing is conducted in the same manner as for gaskets and tapes. In addition, it is necessary to confirm compatibility of the setting block with the silicone edge seal of the insulating glass unit.
Backer rods or folded membranes used to support the silicone weather seal also require compatibility testing as outlined above.
3.13.4 Structural Silicone Sealants
As the name of the system, structural silicone glazing, implies only structural grade silicone sealants are acceptable for structural glazing. Inorganic
silicone sealants are the only materials with the necessary properties of strength, durability and adhesion for use in a load carrying structural role.
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Structural silicone adhesives are produced in one- or two-part formulations. One-part sealants are used for field applied two-sided applications and for glass-to-glass weather seals. Two-part sealants are used in shop applied two- or four-sided applications where particular control of cure time is required. Aside from production issues such as shelf life, tack free time, cure time, gunnability, viscosity, etc., the two characteristics of prime concern with respect to structural silicone are the cohesive and adhesive strengths of the material. The design goal with SSG is to ensure that failure, if it occurs, happens cohesively at a predictable level rather than adhesively at an unpredictable level.
The cohesive strength of the silicone is dependent on the formulation and cure mechanism of the sealant. The strongest sealants are not necessarily the best choice due to their high stiffness and high resultant bond line stresses. Also the cure mechanism of high strength sealants can release volatile compounds into the glazing cavity. Acetoxy cure (vinegar smell) silicones should not be used in structural glazing for this reason.
Adhesive strength is a property dependent on the nature of the sealant but is heavily influenced by the condition and type of surface to which the sealant is applied. Silicone sealants bond tenaciously to clean uncoated glass substrates due in part to the common silica base of the two materials. Adhesive failure, where observed in the field, occurs almost exclusively on the other bonding surface. Contaminants on this surface, along with the type of finish (see Section 3.12), affect the bond.
3.13.5 Fabricated Glass Products Insulating Glass Units
The edge seal of insulating glass units used in structural silicone glazing vary in two important aspects from non-structural silicone glazing. The secondary seal of the unit must be of silicone sealant. Silicone is the only edge seal material that has demonstrated adequate durability (particularly with respect to UV resistance) and strength for this application. The second variance with typical IG units is the width of the secondary seal. The seal must be designed for a defined portion of the wind load, over and above the dimension needed to accommodate normal internal stresses in the IG unit. This usually results in a secondary seal at least 6 mm (1/4 inch) in width.
The combination of silicone secondary seal and a designed width to
accommodate wind loads results in the exclusive use of dual sealed silicone units.
The architectural design intent in the use of SSG is often to present the appearance of larger or continuous lites of glass. The weather seal, IG edge seal and the framing behind creates a dark band at the SSG joint (mullion or rail). The use of standard aluminum edge spacers results in highly visible streaks of light coloured metal at any variation in the width of the primary seal when the wall is in bright sunlight. As such, the use of black anodized spacers is recommended for SSG walls.
Laminated Glass
Laminated glass lites, used either alone or as the inner lite of an IG unit are common in SSG systems. The configurations or constituents of the laminated glass do not change in SSG — however — a caution must be raised regarding the appearance of the edge of the lite at the weather seal where the edge is
exposed to view. Most silicone sealants cause minor isolated delaminations of the vinyl interlayer at the glass edge. These delaminations usually only extend 1 to 3 mm ( 1/25 to 1/8 inch) in from the edge. They are normally not progressive and are of no structural significance. However, the edge
imperfections may become a significant aesthetic issue. Tinted or Coated Glass
Tinting of glass has no effect on SSG. In contrast, coating on glass can have a significant effect on the structural silicone bonding of the glass to the framing and on the edge seal of the unit if the silicone must bond to the coating. If this occurs, the bond of the coating to the glass becomes part of the load path at the unit edge. Given the likelihood of an inconsistent bond of the coating to the glass or of the silicone to the coating, all coatings must be “edge deleted” over the width of the silicone bond line.
At the edge seal of the unit any coating on the inner glass surfaces must also be edge deleted at the bond line of the secondary seal. Also in SSG systems where the edge seal is exposed, coatings should be deleted over the width of the primary seal. Despite manufacturers claims that there is no functional reason to do this, most coatings when in contact with the primary seal reflect light differently and create an aesthetic issue at the glass edge.
3.13.6 Spandrel Glass
Spandrel glass components in an SSG system must comply with all the requirements of spandrel glass in general. In addition, as discussed in 3.13.5, any coating applied to the inside surface of the glass must be edge deleted over the width of the structural silicone seal. Any coating applied to the spandrel glass must also be tested and verified as compatible with the silicone sealant.
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3.13.7 Finishes
As the structural silicone sealant must bond to the finish on the framing member, the finish plays a major role in the integrity of the bond. The silicone must bond to the finish and the finish must bond structurally to the aluminum.
In the first case all aluminum used in SSG systems must be finished. The natural oxide surface of mill aluminum is too variable and contaminated with oils to allow consistent silicone bond.
Anodizing and organic coatings form the principal finishes used with SSG systems. In all cases the adhesion to these finishes must be verified by standard testing. This testing is carried out on production run samples in the laboratory before the finish is approved for use and in the field as a
continuing quality control procedure.
Adequate adhesion can be obtained with both anodized and coated surfaces. However, it is imperative to always check adhesion. It must not be assumed that because the same type and brand of coating is being used and only the colour is different that proper adhesion will be attained. The different pigments used to produce different colours or shades can markedly change the adhesive strength of the silicone sealant. Changes in coating formulation over time, even of the same colour, can also impact adhesion.
SSG and Finishes
Regardless of prior experience with adhesion to a particular coating, laboratory and field adhesion testing of the structural silicone must always be conducted.