Structural Silicone Sealant and Glass Fins

Introduction

Three-sided bonding refers to the potential for a sealant joint, typically rectilinear in shape, to be in adhesive contact on three sides of the geometry which will constrain freedom of movement of the sealant material. Three-sided adhesion is typically viewed negatively, but this may not be negative in all circumstances as a rule of thumb. Finite element analysis was used to explore the stress distribution in silicone joints with three-sided adhesion in different building joints to understand the impact of the constrained movement.

Weatherproofing Joint

Weatherproofing joints are typically referred to as butt joints where the sealant is applied between two substrates. Weatherproofing joints are designed to accommodate movement, typically dimensioned as small as possible with the widest movement available based on movement capability ratings of the sealant. Backer materials used to prevent three-sided adhesion interaction are usually soft and fitted via compression. If the backing material is not soft where three-sided adhesion could become a concern, then bond breaker materials are used to release the sealant from the third side.

Unique weatherproofing joint geometries, one based on an actual proposed design, were analyzed based on different variables affecting the potential for three-sided adhesion. One was a joint with complete freedom and no backing material interaction. Second was a joint with a metal iron profile as a backing material with a slight gap between the metal and substrates. Third was the same joint, but sealant was indicated as having filled into the small gaps between the substrate and metal. Figure 1 illustrates the different joint shapes.

Figure 1: Unique Joint Geometries

Joint movement simulation was 1.5mm which corresponds to 12.5% movement capability which is the smallest designation for ASTM C920. The initial simulation utilized a two-part structural sealant with a movement capability of 25% that could be used for both structural and weathersealing applications.

Finite element analysis indicated a peak strain of 24% for the unbound condition and 72% and 223% for the bound conditions. The peak strain associated to initiation of fracture for the sealant used in analysis is approximately 100%, so the peak strain for the condition with three-sided adhesion and fully filled cavity along the sides indicates a high probability of potential failure.

(a) Silicone Joint without Bonding to Backer Rod (2side bonding) 24% strain

(b) Silicone Joint Bonding to Backer Rod in Three-Sided Bonding, 72% strain

(c) Silicone Sealant Bonding with Bonding on All Interfaces in Three-Sided Bonding, 223% strain

Figure 2: FEA Output of Unique Joint Geometries

Additional sealants were evaluated to understand if the differences in mechanical behavior could change the development of peak strain in the three-sided condition with sealant along the face of the metal. Behavior models for two silicone weatherseal were used in the simulation. Table 1 shows the material model parameters and effective modulus at 50% strain for a tensile adhesion joint.

Table 1: Material Model Parameters of Analyzed Sealants

(a) 2K Structural Sealant with Steel Backer Rod in Three-Sided Bonding

(b) Weathersealant 1 with Steel Backer Rod in Three-Sided Bonding

(c) Weathersealant 2 with Steel Backer Rod in Three-Sided Bonding

Figure 3: Analysis of Variable Sealants with Steel Backer Rod

Peak strains for each sealant were around 70% regardless of the formulation, see figure 3. The magnitude of peak strain for this condition would be within the working ability of the sealant based on laboratory testing. However, the movement of 12.5% is half or a fourth of the statement movement capability of the sealant indicating a reduction in capability based on the geometry.

Three-sided bonding in structural silicone glazing applications

A common detail with potential for three-sided adhesion in structural silicone glazing are butt joints in a fin or total vision systems where three glass plates are connected by a singular silicone sealant joint, see Figure 4.

Figure 4: Typical fin glazing detail

A fin assembly where the glass was captured mechanically and structurally glazed along the vertical intersection was simulated under a negative windload. The glass panels were varied by height from 2200 mm to 6000 mm with a standard width of 2200 mm short span. The fin was 254 mm deep by 39.44 mm wide. Shown in Figure 5. A windload of 2.80 kPa was used in the simulations.

Figure 5: Drawing of fin glazing used in analysis

Under the negative 2.8kPa wind load, maximum glass panel deflection ranges from 14.7mm to 22.06mm, and peak strain in the sealant ranges from 14.45% to 15.48%. See Figure 6 and 7. This demonstrates that structural sealant in a three-sided bonding configuration can provide sufficient support and stiffness to limit the glass deflection without developing significant amount of strain even in a complex case of multi-sided adhesion.

Figure 6: Analysis Output for 6000 mm Wide Lite

Figure 7: Analysis Output of 2200 mm wide lite

Conclusion

Three-sided adhesion should be avoided in applications which intend to maximize the available movement between substrates. If a rigid backing material or specific joint geometry creates a potential for the increased rigidity of multi surface adhesion contact, finite element analysis can provide insight as to the impact of the reduction in movement capability. In structural applications, increased rigidity due to multisurface adhesion contact may be beneficial. For structural connections, reducing the amount of movement intentionally in a joint may be beneficial to the sealant and overall performance of the assembly when properly designed. As such, using the generality of avoiding three-sided adhesion in all applications should be redirected back to those that intend to maximize the movement.

ASTM C719 Standard Test Method for Adhesion and Cohesion of Elastomeric Joint Sealants Under Cyclic Movement (Hockman Cycle), ASTM International, West Conshohocken, PA, 2014, www.astm.org

ASTM C920, Standard Specification for Elastomeric Joint Sealants, ASTM International, West Conshohocken, PA, 2014, www.astm.org