How to Build a Better Flashing Tape

UV-cured acrylic adhesives are enabling the next generation of high-performance flashing tapes.

By Kelly Patrick, Senior Product Manager, Shurtape Technologies, LLC

Beneath all the finishings and furnishings that make a house a home or an office building a great place to work, building envelope products are working to protect the structure, help it achieve optimal energy efficiency, and ensure occupant comfort. They fulfill these objectives by blocking moisture from entering the structure and reducing air movement to minimize heat loss or gain, depending on the outdoor environment.

Self-adhering flashing tapes work in concert with products like roof underlayment and water-resistive barriers (commonly called housewrap) to help ensure that a structure’s building envelope can maintain the desired interior environment while providing protection from elements outside. To that end, flashing tapes effectively serve as gaskets that are used around the fenestration, air vents, and pipe penetrations that are common culprits for moisture ingress and air movement.

Flashing tapes also come into play in transition areas where there is an elevation change between two parts of a structure. Examples include the area where a garage abuts a house and where walls transition into a roof. Flashing tape acts as a self-adhering membrane (SAM) on the interface between the two areas, creating seal continuity.

Ideally, flashing tapes are easy to install, effective in a wide temperature range to accommodate various climates, able to adhere to common construction substrates, and durable enough to function effectively across a long lifespan. It can be difficult to achieve all of these goals, however, as adhesive chemistries are not one-size-fits-all solutions. Some adhesives, for example, may provide durability but struggle in high-temperature environments.

Building a flashing tape that covers all the performance benefits mentioned above may be difficult, but it is certainly not impossible. Rubberized asphalt and synthetic rubbers (including butyl rubbers and block copolymers) have long been used for flashing tape application with mixed success. UV-cured acrylic adhesives, however, offer the ideal combination of properties to create the next generation of high-performance flashing tapes.

Traditional Limitations

Historically, rubberized asphalt has been the most common adhesive used in the manufacturing of flashing tape. While it provides a lower-cost option, rubberized asphalt’s limited effective temperature range of just 40-180˚F (4-82˚C) leaves it vulnerable to failure in both cold and hot environments.

Another drawback is that rubberized asphalt incorporates solvents that are not only potentially hazardous but also incompatible with some solvents used in other building materials. This often results in performance issues when rubberized asphalt flashing tapes come in contact with many of the solvent-containing sealants and caulks that are commonly used in construction projects.

Rubberized asphalt also contains volatile organic compounds (VOCs), which dry out the adhesive as it off-gasses over time, negatively impacting the tape’s long-term durability and compatibility with other building components. In addition, VOCs are known to cause air quality problems during and after installation.

Butyl rubbers and block copolymers, on the other hand, both offer improved performance over rubberized asphalt. These synthetic rubbers don’t include solvents or VOCs, and they are commonly effective at 30-200˚F (-1-93˚C), providing a slightly wider temperature range than rubberized asphalt.

However, the tackifiers included in both types of synthetic rubber formulations migrate over time, leading to problems with durability. Both types degrade with long-term heat and chemical exposure, and they darken and discolor as they oxidize, particularly in higher temperatures. In addition, block copolymers suffer from low resistance to ultraviolet (UV) light.

UV-Cured Acrylic

UV-cured acrylics represent a step change in adhesive performance for flashing tapes. The key resides in their highly crosslinked nature. The bonds from crosslinking polymer chains result in strong, durable adhesion to all common construction substrates, including glass-faced gypsum, oriented strand board (OSB), concrete masonry units (CMUs), stucco, and plywood—without the need for primers. Primerless adhesion is a critical time and budget saver that allows building crews to skip primer application and the accompanying wait on dry time, which can vary depending on temperature and weather conditions.

UV-cured acrylics are also resistant to UV light, chemicals, and oxidation. A key performance attribute of these adhesives is their broad operating temperature range—from 0 to over 240˚F (-18-115˚C)—providing the best performance in both cold and hot climates and lengthening the building season into the colder months. In addition, UV-cured acrylics do not contain any solvents or VOCs, which eliminates all of the associated health, environmental, and long-term durability concerns.

Another installation benefit of UV-cured acrylic adhesives is their workability. Unlike rubberized asphalt and synthetic rubbers, UV-cured acrylic flashing tapes are repositionable for a limited time during the installation process. If the tape’s initial application needs to be adjusted, the installer can simply peel it up and reapply it to ensure best performance. This is simply not possible with tapes incorporating traditional rubberized asphalt and synthetic rubber adhesives; their removal can be difficult, and the used tape must be disposed of.

Moreover, the properties of UV-cured acrylics can often be customized to suit specific OEM manufacturer needs, even down to the product level for particular flashing tape application requirements. For example, the thickness can be adjusted to suit rough substrates such as OSB. With specifically formulated properties, the adhesive is able to fully flow into the cracks of rough substrates and maintain a strong bond.

Long-Term Strength and Durability

The adhesives used to produce flashing tapes must be able to provide effective bonds for decades, often under harsh and variable conditions. Additional challenges arise due to temperature fluctuations that result from seasonal weather changes, which cause all of a building’s materials to make constant tiny movements. These slow shearing forces can lead to failure if the adhesive is not simultaneously strong enough to maintain its bond and pliable enough to flow with these movements.

Various methods are available to test adhesive properties and evaluate a tape’s performance. Peel testing is widely understood in the construction industry as a measure of a tape’s ability to adhere to a surface, but one method that deserves increased attention for flashing tapes is shear testing. Generally speaking, shear is a reflection of the tape’s cohesive strength against both push and pull forces. Think of this as the tape’s ability to hold itself together and resist delaminating as wind and temperature fluctuations create building movement over time.

The high degree of crosslinking that UV-cured acrylics achieve during curing leads to extremely strong polymer chains, even at elevated temperatures. This translates to high shear performance and long-lasting cohesive strength that softer adhesives like rubberized asphalt and synthetic rubbers cannot attain.

As previously noted, rubberized asphalt formulations are not able to maintain the required pliability and strength, particularly at higher temperatures, due to their material makeup and VOC content. A recent study examined the shear strength of UV-cured adhesives in comparison to synthetic rubbers, which do not contain these materials, using six different tapes.

Two flashing tapes for each of the three adhesive chemistries (UV-cured acrylic, butyl rubber, and block copolymer) were studied at both room temperature (73˚F/23˚C) and high temperature (149˚F/65˚C). Prior to shear testing according to ASTM D3654 (Standard Test Method for Shear Adhesion of Pressure-Sensitive Tapes), the tapes were all immersed in water for seven days and then stored at 179˚F for seven days.

The tapes were applied to different types of weighted vertical substrates, and the length of time each tape lasted until failure was recorded. Details and results are shown in Table 1.

As shown, the UV-cured acrylic adhesives clearly outperformed their synthetic rubber counterparts. UV-cured acrylics provide exceptional shear strength, dominating both butyl rubbers and block copolymers at room temperature and particularly at elevated temperatures.

Next-Generation Capabilities

When designing and manufacturing a flashing tape, it is vital to take the entire balance of adhesive properties into account. UV-cured acrylic adhesives outperform both rubberized asphalt and synthetic rubbers in long-lasting durability, operating temperature, and surface compatibility while offering unique capabilities such as custom formulation and tape repositionability. Flashing tapes that incorporate UV-cured acrylic adhesives represent next-generation products, providing building envelope manufacturers, architects, contractors, and builders with high-quality, must-have performance options.