It’s Complicated: Sealant Selection

By Rick Rinka

The primary mission for sealants includes integrating fenestration frames and mounting flanges with a building’s drainage plane, as a part of roof-to-ground weather-resistant barriers (WRB). In doing so, a sealant isn’t intended to prevent, but to accommodate, fenestration joint movement caused by thermal expansion and contraction, windload, the settling of the structure, operation and other factors. Meanwhile, the sealant must resist degradation from exposure to water and ultra-violet (UV) radiation over its service life. Consideration must also be given to the type of substrate to which a sealant must adhere, joint configuration and the amount of joint movement to be expected, as well as the compatibility of sealants with other materials. In other words: it can be complicated.

Fortunately, a good overview and primer is provided by the newly updated AAMA 851-20, Fenestration Sealants Guide for Windows, Window Walls and Curtain Walls.

A Look at the Factors

Joint configuration and action are major considerations when selecting a sealant system. Once the type of joint between two parts is determined (butt, lap or bedding), the dimensions of the joint change, due to relative movement of the joined parts, must also be assessed, as this is critical to the selection and use of the proper sealant. These are depicted graphically in 851-20, which also defines criteria.

Sealant compounds are classified according to the amount of extension and compression movement they can withstand without failing, including:
• Low-cyclic movement: sealants with little movement capability (less than 5%) are used where little or no movement is expected;
• Medium-cyclic movement: with sealants that have a movement capability of 5% to 12.5%; and
• High-cyclic movement: sealants with cyclic movement capabilities of more than 12.5% are recommended when high joint movement is expected.

Temperature-induced expansion and contraction of the joining parts are shown in diagrams. The overall temperature differential can be used to calculate the maximum expected expansion and contraction of a joint, using a step-by-step calculation method and graphic calculators (nomographs) provided in AAMA 851. Based on the length of the joining parts, a nomograph additionally indicates the proper design width of the joint for different classes of sealant.

There are many types of sealants available to meet various joint conditions, but in general these boil down to wet, dry and combinations for both working and non-working joints.

In addition to movement capability according to classification, the most critical properties to consider in the selection of sealant type for a given project are:
• Adhesive strength—depending on the substrate material, a primer may be required;
• Cohesive strength—a material that lacks the strength to “stay together” under repeated stress cannot provide a suitable seal;
• Movement capability;
• Hardness (stiffness, or resistance to deformation under load);
• Elastic recovery (after deformation);
• Permanent set (after stretching);
• Compression set (after compression);
• Modulus (tensile strength at specified extension);
• Stress relaxation (stretching without increased internal stress);
• Durability under the effects of weathering; and
• Compatibility among sealants that may contact each other (without embrittlement, softening, increased tackiness, bleeding, change of color, lack of cure, or loss of adhesion).

These characteristics can be quantified, enabling comparison among sealant types and brands, through the use of ASTM and other published laboratory test methods noted in 851-20.

Rich Rinka is technical manager, fenestration standards and U.S. industry affairs for the Fenestration and Glazing Industry Alliance (FGIA).

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DWM Magazine

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