The primary building sealant problem is that no method exists to provide high quality data for elucidating the relative importance of the k = 4 major weathering factors:
X3: ultra-violet radiation, and
X4: mechanical loading
(and interactions) on the change in pre-failure sealant mechanical properties. A critical attribute of an effective sealant is the ability to span and seal gaps between dissimilar building materials. These polymeric materials experience daily (~±7%) and yearly cycles (~±25%) of strain deformation. As these materials are exposed to the weather, molecular changes occur that eventually prevent the sealant from responding to these imposed strains, leading to failure of the sealant. Characterizing these molecular changes and attributing them to specific exposure factors and conditions will enable the development of models to predict in-service performance. Improvements in sealant material performance and durability will then enable the development of sealant products having longer service life, significantly-decreased maintenance/repair costs, and ultimately result in increased sustainability.
Measuring the mechanical properties of sealant materials is intrinsically challenging due to their inherent non-linear viscoelastic nature. Assessing the molecular changes in the sealant requires straining the sample and measuring the stress response to obtain the sealant modulus. The measured modulus is dependent on specific strain level, time required to impose the strain, and strain history. While a sealant experiences a strain event--either during testing or within the in-service environment--it is also relaxing those imposed strains on time scales ranging from minutes to months. As the sealant is constantly changing and responding to applied strain, assigning change in modulus to the environmental factors is a considerable technical challenge.
Currently used in-field and in-lab methods to assess sealant durability do not have the capability to predict performance. These methods generally fall into two categories:
1. Threshold based methods (ASTM C719) which impose a series of environmental exposures after which the sealant is visually evaluated, and
2. Multi-year outdoor exposure tests evaluated with visual inspection.
Further, such visual evaluation fails to yield an understanding of the essentially non-linear viscoelastic modulus, or tan appreciation of the molecular changes occurring during exposure that precede failure. Applied strain and the corresponding stress, which are critical components to the sealant performance, are not monitored by these current methods.
The BFRL at NIST is perfectly attuned to attack this engineering problem because
1. the problem is precisely aligned with BFRL's strategic goal of "Sustainable Infrastructure Materials",
2. BFRL has a unique, controllable, in-lab weathering facility (the NIST SPHERE),
3. BFRL has the personnel with the experience, expertise, and familiarity with rheological characterization of elastomeric materials, and
4. BFRL has pre-existing strong relationships with the sealant industry.
The Statistical Engineering Division within NIST's ITL is perfectly attuned to collaborate with BFRL on this problem because
1. It is SED's charter mission to be of ready-assistance to NIST mission-critical projects
2. ITL/SED has the experiment design, data analysis, and modeling expertise to efficiently carry out and guide the statistical components of this problem.
The NIST/BFRL/ITL solution to this problem involves
1. developing precise modulus-measurement instrumentation, characterization and exposure protocols,
2. developing appropriate experimental sampling plans,
3. developing multivariable databases,
4. applying insightful statistical data analysis tools (primarily graphical), and
5. developing quantitative mathematical models.
A series of experiments were carried out to assess the effects, the interactions, and the optimal settings of the 4 principal factors considered to be most critical to the aging of sealants.
Finally, by designing and executing stress-relaxation experiments at NIST to characterize the non-linear viscoelastic modulus of the sealant before and after exposure, and by carefully controlling and monitoring the above 4 factors, molecular changes within the sealant can be correlated to the specific exposure conditions.