Various glazing accessories (gaskets, setting blocks, cap plugs, spacers, and tapes) are used in the assembly of commercial curtain walls, window walls, storefronts, and other applications. These products are often in close proximity or direct contact with the silicone sealants used on the fenestration system. Chemical interactions can occur between these components that can result in discoloration or even loss of adhesion. Understanding these interactions and adequate testing can prevent unwanted failures and the costs associated with them.
Gaskets, setting blocks, and tapes are all made from different base chemistries and manufacturing methods. Setting blocks and pre-formed gaskets are often made from EPDM, neoprene, silicone, or other elastomers and are typically manufactured on an extrusion line. Additives in the form of plasticizers and oils can be incorporated into the formulation to help facilitate smooth processing.
Tapes are often produced on an adhesive coating line where the liquid adhesive is coated onto a release liner or other substrate and then dried and “cured” to initiate cross-linking for improved physical performance properties. The adhesive can then be laminated onto a foam core to form the tape. While plasticizers and oils are not typically part of the adhesive formulation (especially for the acrylic adhesives) they can be present in the foam and there can be very low levels of unreacted monomer or other low molecular weight species present in the adhesive.
Silicone sealants are formulated from proprietary variations of PDMS (polydimethylsiloxane) along with fillers, silicone fluid, and small amounts of solvent to help with adhesion and to keep the sealant “gunnable” in various environmental conditions. When the sealant is applied on or near any setting block, gasket, or tape in a tight space and especially in warm conditions, the plasticizers, oils, solvents, and other chemicals will begin to interact and that’s where trouble can begin.
Testing for Compatibility
The primary test method used to evaluate sealant compatibility with glazing accessories is ASTM C 1087 “Standard Test Method for Determining Compatibility of Liquid-Applied Sealants with Accessories Use in Structural Glazing Systems”.
This test method is a laboratory screening procedure that involves placing a reference sealant and the “test sealant” against the accessory on a glass plate. Control plates without the accessory are also prepared. The plates are conditioned for 1 week at standard room temperature conditions and then exposed to UVA lamps at 340nm wavelength for 21 days. The temperature at the surface of the test panels must be kept at 118°F +/-3.6°F and the lamps are rotated or changed to maintain steady UV intensity. This set of conditions induces molecular mobility and if there are interactions between the components, they will tend to show in this type of test.
Once the UVA exposure is completed, there are three primary observations to be recorded:
- Changes in color of the sealant
- Changes in adhesion of the sealant to the glass
- Changes in adhesion to the accessory being tested
In structural sealant glazing, the sealant acts as the primary means of bonding and can also be the primary weather seal. The integrity of this bond is critical to the overall lifetime performance of the system. Any degradation observed in this test with discoloration or loss of adhesion is not permitted to gain a structural warranty.
Discoloration on its’ own does not imply complete failure. The test sealant may be able to be used as a weather seal but usually a darker colored version of the sealant will be recommended.
Silicone sealant manufacturers can be very helpful with this process. They have the lab, staff, and equipment to conduct this test and to provide the feedback needed to make the best material choices for long term performance.
Any structurally glazed curtain wall, window wall or storefront acts as a system. Multiple components make up these systems and it is important that these components are compatible and “play nicely together”. The structural silicone sealant is doing the heavy lifting in holding everything together and acting as a barrier to water infiltration. It is important that any accessories used to complete the fabrication are compatible with the structural silicone. ASTM C1087 offers a quick and inexpensive way of forcing interactions between the glazing accessories and the structural sealant. Taking the time to run compatibility testing can help to prevent costly rework and help to ensure long term performance.
Special thanks to Brent Dull at The Dow Chemical Company for his assistance.