How Long Will It Last?
Predicting the Durability of Building Construction Sealants
Collaborative work housed at the National Institute of Standards and Technology is modeling sealant service life and enabling new ASTM standards development. The project will also impact other polymer applications in the future.
The unique properties of polymers make them attractive as materials: They adhere, stretch, insulate and flow.
People use these materials to separate the inside of buildings from the outside, keep us dry when it rains, protect us from threats such as flying glass in wind storms, conserve energy by insulating walls and stopping air leaks, and make our world safer with polymer fiber bullet-resistant vests and tough airbags.
These same properties that make polymers so attractive in diverse applications are changed over time by the weather. For more than 100 years there have been efforts to predict how the properties of plastics change with exposure to weather, but with little success.
Now, a project under way at the National Institute of Standards and Technology is progressing toward new testing equipment and validated predictive models for certain polymers used in construction - that is, sealants - and related standards are being developed in ASTM International Committee C24 on Building Seals and Sealants.
Choice Materials, Complicated Problems
Predicting sealant service life is not easy.
Sealants, by their very formulas, are complicated materials. Manufacturers combine a base polymeric material with numerous additives and fillers, then adjust components to enhance one property or another. Such formulations make for complexities in the final product, hence the difficulty of predicting service life.
In construction applications, sealants join dissimilar materials; they fill the space between glass or aluminum window frames and the building envelope, attach glass to steel or stone, or aluminum to steel, or close gaps in other places within the building envelope. Each material responds differently to changes in the weather, which exerts a variety of influences on the sealant.
Should the sealant in a building fail and allow the intrusion of air and water, it can have a widespread and costly impact even though sealants account for just a small percentage of the materials in the structure. Field studies in the literature indicate that 50 percent of building sealants fail within 10 years, and 95 percent fail within 20 years. In the United States, tens of billions of dollars are spent each year on home repairs, mostly due to water intrusion.
Adding further complexity to sealant service life prediction is the difficulty of understanding and quantifying the effects of weather. Natural factors such as temperature, humidity, radiance and mechanical loading, identified by researchers in the 1990s for their connection to sealant durability and with known associated mechanisms, vary both themselves and in relation to each other.
Other factors also apply. Outdoor weathering has been the standard way to test sealants, but that takes time and its repeatability is debated. Otherwise, no comprehensive methodology currently exists to predict sealant failure, and research to change that situation does not exist in universities or other organizations outside NIST. And company studies tend to focus on their own products and the results are kept in-house.
Yet work is advancing to address the situation.
In 2000, the Service Life Prediction of Sealant Materials Consortium was organized in NIST's Engineering Laboratory; the EL mission is to meet the measurement science, standards and technology needs of the U.S. building industry. The collaborative effort represented by the consortium was motivated by the need to develop and provide reliable service life prediction tools for the sealant industry.
The consortium includes base material formulators, additive suppliers, packagers and sellers. The 10 largest global sealant manufacturers have been or are part of the consortium, as is the Forest Products Laboratory.
The consortium is furthering the understanding of sealant durability and how to assess it. The project addresses:
- The development of characterization protocols for sealant modulus, which will enable the linkage of exposure conditions to changes within the material;
- Monitored outdoor exposure that includes applied strain, which will contribute to exposure database development and to predictive models;
- Controlled indoor exposure device improvement to characterize sealant modulus before and after controlled weathering exposures; and
- The development of databases with exposure results to develop predictive models.
Progress is being made in validating the significance of temperature, humidity, ultraviolet radiation and mechanical load in weathering; in developing standards in ASTM Committee C24; and in the development of a powered outdoor weathering sealant testing devices to gather data that will enable validation against a predictive model. Of particular use in the NIST project, in addition to the available technical knowledge and sealant industry involvement, is the SPHERE (simulated photodegradation by high energy radiant exposure). The SPHERE, a unique piece of equipment at NIST, delivers highly uniform temperature, humidity and ultraviolet radiation so that useful data can be generated when its environmental chambers apply variable forces in testing samples.
ASTM C24 Standards
Tracking modern construction sealant evolution has been the work of ASTM Committee C24. Formed in 1959, Committee C24 consists of subcommittees that consider sealants according to standard type as well as specific attributes or sealant types. The committee has developed close to 100 standards, and among these documents are general test methods as well as those for adhesion and weathering, plus guides for sealant use and evaluation.
To predict sealant service life, standard methods for characterization and exposure are needed, as is a predictive model. Standards that begin to meet these needs have been developed by C24, and as the technology and equipment improve, the standards continue to be updated.
Among C24's significant current sealant weathering standards are the following:
- C1735, Test Method for Measuring the Time Dependent Modulus of Sealants Using Stress Relaxation, characterizes a sealant by using a test rather than visual inspection; the test consists of two loading cycles followed by a stress relaxation procedure. The method can be used to screen building joint sealant performance because modulus helps indicate a sealant's ability to withstand building movement.
- C1519, Test Method for Evaluating Durability of Building Construction Sealants by Laboratory Accelerated Weathering Procedures, details a weathering process for samples that adhere to contact surfaces. Following curing and artificial weathering, samples are subjected to extension and compression, and then examined for flaws in a process that allows the user to evaluate and compare the durability of sealants.
- C719, Test Method for Adhesion and Cohesion of Elastomeric Joint Sealants Under Cyclic Movement (Hockman Cycle), describes an accelerated laboratory procedure that enables the user to evaluate the performance of sealants subjected to water immersion, cyclic movement and temperature change. The procedure reflects the common cohesive failure by the sealant or the adhesive failure between the sealant and the substrate, or both.
Additional proposed standards now under development in Committee C24 detail a fatigue resistance test, comparing sealant behavior to reference photographs, evaluating installed joint sealants and more.
More Uses than Building Sealants
The sealants research and related standards won't be limited to building sealants. Other polymer applications will benefit as well.
Ultraviolet light and moisture affect the durability of ballistic fibers, which are the essential polymeric material in bullet-resistant vests, and the ability to predict service life will be useful for such equipment. Photovoltaic units have many polymeric components that must function reliably for many years and resist degradation by moisture. A separate consortium is being formed at NIST to address PV materials durability.
Standards will also be developed that apply to predicting the service life of polymers for these applications as well as bridge coatings, firefighter coat linings, airbag materials and more. In the case of building sealants, perhaps it won't be too long before sealants will be designated by service life so that savvy shoppers can choose a product based on performance.
Christopher C. White, Ph.D., is a research chemist in the Polymeric Materials Group of the Materials and Construction Structural Systems Division, which is part of the Engineering Laboratory at the National Institute of Standards and Technology. He is a member of Committees C24 on Building Seals and Sealants, E06 on Performance of Buildings and E54 on Homeland Security Applications.