Service Life Prediction of Sealant Materials Consortium

Summary

This research will deliver critical measurement science for predicting sealant service life (a significant part of the US building industry) and continue the global leadership in service life prediction performance-based standards for sealant materials. In this project, high precision sealant mechanical property data will be generated from laboratory and field exposures as a function of ultraviolet radiation (UV), temperature, moisture and mechanical loading for use in a reliability-based methodology for service life prediction.  

Description:

Objective:  Develop, implement, and transfer reliability-based service life prediction tools for sealants capable of relating field and laboratory exposure results to the sealants industry, by 2013.

What is the problem? Nationally, homeowners spend >$60B/yr repairing their homes.[1] Insurance studies show that water damage is 10 times more likely than fire, with 28% of all property claims related to water-related damage.[2] Field studies indicate that 50% of sealants fail within 10 years, and 95% fail within 20 years.  Currently, no methodology exists to predict failure of sealants.  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 imposed strains, leading to failure of the sealant.  When undetected, this bulk water intrusion and air infiltration leads to significant preventable repair cost and energy loss.  Characterizing these molecular changes and attributing them to specific exposure conditions enables the development of models to predict in-service performance. Resulting improvements in sealant performance, known durability result in decreased maintenance and repair costs, and ultimately increased sustainability.

Why is it hard to solve? Dissimilar building materials expand and contract at different rates when exposed to thermal or humidity cycling.  Polymeric sealants are used to seal these ever changing gaps.  Sealant materials respond to the imposed changes in gap dimension (strain) through a variety of mechanisms.  Some sealants adopt a new equilibrium dimension (decreasing the stress) through viscous dissipation of the imposed strain.   Other sealants elastically deform and keep a high stress level. These are two of the many mechanisms observed for sealants responding to changing gap conditions.  The sealant will fail when the molecular changes no longer enable the sealant to respond to the imposed deformation.  The short range modulus (stress/strain) of a sealant changes with time, imposed stress level and history of strain.  As these sealant materials are exposed to a suite of environmental variables (temperature cycles, moisture changes, UV radiation, and mechanical deformation) the molecular chemistry will change and thus the sealants ability to respond to imposed strain will change.  Predicting when those molecular chemistry changes will result in macroscopically observable tearing, delamination, or loss of adhesion requires knowledge of how the components of the environment change the molecular chemistry.  The macroscopic failure may take years to observe, but the underlying molecular changes occur on a much shorter time scale. Quantifying these chemical changes and then attributing them to specific environmental factors and then relating that to macroscopic failure presents a series of significant technical challenges. 

How is it solved today, and by whom? Currently used methods to assess 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) Multiyear outdoor exposure tests evaluated with visual inspection.  Visual evaluation does not give an understanding of the non-linear viscoelastic modulus or 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 in these current methods.

Why NIST?  This research is linked to the mission of EL by anticipating and meeting the measurement science, standards and technology needs of the US building industry.  This research is aligned with EL's strategic goal of Sustainable Infrastructure Materials, and Net Zero Energy Buildings.   This research utilizes the core competency of EL's unique weathering facilities (NIST SPHERE), personnel with expertise in rheological characterization of elastomeric materials, and strong relationships with the sealant industry. 

What is the new technical idea? Combining a reliability-based approach with a cumulative damage model has been successful in the agricultural, photographic, imaging, and radiation exposure industries for generating dose-damage data that enable development of predictive models.  These models, both statistical and analytical, have been used to confirm common mechanisms of degradation in both the laboratory and outdoor exposures.  The new technical idea will build upon these successes and apply these approaches to the prediction of sealant in-service performance.  This will involve developing precise instrumentation, characterization and exposure protocols, multivariable databases, and quantitative mathematical models. Four principal factors are considered critical to the aging of sealants: temperature, humidity, ultra-violet radiation, and imposed mechanical strain. It is critical in the laboratory to control and monitor each of these factors.  By using a stress-relaxation experiment to characterize the non-linear viscoelastic modulus of the sealant before and after exposure, molecular changes within the sealant can be correlated to the specific exposure conditions.

Why can we succeed now?  EL possesses unique weathering facilities (NIST SPHERE), personnel familiar with rheological characterization of elastomeric materials, and strong relationships with the sealant industry.   The NIST SPHERE can deliver highly uniform temperature, humidity, and ultra-violet radiation.  Modification of the environmental chambers to apply strain to multiple samples has recently been accomplished.   Additionally, outdoor exposure instruments have been developed to impose dynamic strain during the exposure.

What is the research plan? This project will address four measurement science needs: 

(1) Development of characterization protocol for sealant modulus:  This measurement need will motivate the development of a characterization method that allows precise, repeatable determinations of the sealant modulus, which will enable the linking of sealant exposure conditions to the kinetics and mechanisms of molecular change within the sealant.  Understanding how the long term exposure to strain affects the sealant properties and dimensions is an ongoing measurement challenge to be addressed in 2011. During extended outdoor exposure some sealants adopt a "permanent" set. These sealants will be forced back to the initial conditions for periods of time and characterized with stress relaxation measurements.   It is critical to develop and use a method such as stress relaxation to account for the changes in the viscoelastic nature of the sealant.  The exposure will affect the complex modulus of the sealant. 

(2) Monitored outdoorexposure that includes applied strain:   The second major measurement science need is to develop outdoor exposure protocols where mechanical strain is applied to the sealant.  For commercial structures, the sealant is strained proportionally to changes in temperature.   Based on the results obtained from sealant materials tested on these devices, it is clear that a significant measurement need is test devices that can apply strain to, as well as characterize the mechanical properties of sealant during the outdoor exposure. These devices will be a major focus of the FY2011 efforts, and will enable the in-situ collection of modulus data on the sealant exposed to the outdoor weathering.  This data is critical to the development of exposure databases and predictive models.

(3) Controlled indoor exposure that includes applied strain: The third measurement need is the improvement of indoor exposure devices used on the SPHERE that can actively characterize the modulus of sealant materials proceeding and following controlled exposure to UV radiation, temperature, moisture and applied strain.  For FY2011, a the newly developed capability for sub-ambient exposure temperatures (down to 5 ^oC) will be incorporated into the research plan.

(4) Development of databases and resulting models with predictive capability:  The fourth measurement need is for databases of exposure from both indoor and outdoor data sources, which will be used to develop models with predictive capability.  The model development will require the services of outside academic collaborators who were successful in developing a similar model for clear epoxy coatings. Preliminary model development and statistical analysis will proceed in FY2011 based on the availability of funds.

How will teamwork be ensured?  The team is composed of members of the Polymeric Materials Group.  Leveraging the SPHERE and similar systems will enable significant teamwork.  Additionally, the ten largest global sealant manufacturers, the Forest Products Laboratory, and the Adhesives and Sealant Council are all actively involved in this project.

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[1] http://www.census.gov/const/C50/histtab1.pdf [2] http://www.insurancejournal.com/news/national/2010/09/17/113337.htm

Major Accomplishments:

Recent Results in FY 2010: 

Industrial workshops

October 29-30 2009, Consortium oversight review meeting, Gaithersburg, MD

April 16-17 2010, Consortium oversight review meeting, Gaithersburg, MD 

Peer reviewed papers:

C.C. White, "Studies on the Effect of Movement During the Cure on the Mechanical Properties of Building Joint Sealant," Polymer Engineering and Science, Vol 50, Issue 1, January 2010, p 113-119.

Karul A, Tan KT, White CC, Hunston DL, Marshall ST, Akgun B, Satija SK, Soles CL, Vogt BD,  Impact of polymer modulus/chain mobility on water accumulation at polymer/metal oxide interfaces, POLYMER, Volume: 50, Issue: 14,  Pages: 3234-3239,  JUL 3 2009

C.C. White, D. Hunston and K.T. Tan, Does imposed strain affect modulus change in sealant subjected to outdoor weathering, Journal of ASTM International, Volume 6, Issue 2, 2009

K.T. Tan, C.C. White, and D.L. Hunston, An adhesion test method for spray-applied fire resistive materials. Fire and Materials, in press.

K.T. Tan, C.C. White, D.J. Benatti and D.L. Hunston, Effects of ultraviolet radiation, temperature, and moisture on aging of coatings and sealants: A chemical and rheological study, Polymer Degradation and Stability, online June 2010.

Book chapter

C.C. White, D.L. Hunston and K.T. Tan, Effect of strain on the modulus of sealants exposed to the outdoors, in: Durability of building and construction sealants and adhesives: 3^rd symposium, A.T. Wolf (Ed.), ASTM STP 1514, ASTM International, West Conshohocken, Philadelphia (2010).

Proceeding papers

C.C. White, D. Hunston and K.T. Tan, Predicting the In-service performance using the NIST SPHERE, the 4^th European Weathering Symposium EWS, Sept. 16-18, 2009, Budapest, Hungary.

D.L. Hunston, K.T. Tan, S.K. Satija, C.C. White, B.D. Vogt, C. Clerici, Moisture attack on adhesive joints: Role of adhesive and interface properties, American Society for Composites Annual Meeting, Symposium Honoring Albert Cardon, Sept. 15-17, 2009, Wilmington, DE.

C.C. White, D.L. Hunston and K.T. Tan, The effect of prestrain on non-linear modulus characterization of filled elastomers, Proceedings of the 81^st Annual Meeting of the Society of Rheology, Oct. 18-22, 2009, Madison, WI.

D.L. Hunston, K.T. Tan, B.D. Vogt, S.K. Satija, C. Clerici and C.C. White, Little things mean a lot: Water and the adhesive bond, Proceedings of the 33^rd Annual Meeting of the Adhesion Society, Inc., Feb. 21-24, 2010, G. Anderson (Ed.), pp. 4-6 Daytona Beach, FL.

K.T. Tan, C.C. White, D.L. Hunston, K. Steffens, T. Hatlee, K. Hamilton, B.D. Vogt and J.W. Chin, Roles of adhesive and interfacial properties on humidity-induced failure, Proceedings of the 33^rd Annual Meeting of the Adhesion Society, Inc., Feb. 21-24, 2010, G. Anderson (Ed.), pp. 186-188, Daytona Beach, FL.

C.C. White, K.T. Tan and D.L. Hunston, Development of adhesion test methods for spray-applied fire resistive materials, Proceedings of the 33^rd Annual Meeting of the Adhesion Society, Inc., Feb. 21-24, 2010, G. Anderson (Ed.), pp. 201-203, Daytona Beach, FL.

C.C. White, K.T. Tan and D.L. Hunston, Predicting the in-service performance of materials exposed to outdoor weathering, the Technical Seminar Proceedings of the 2010 week of learning Pressure Sensitive Tape Council – Tech 33, May 12-14, 2010, Las Vegas, NV.

Continued CRADA agreements with nine sealant companies for Phase III (focus of Phase III is the technical plan outlined above), in the process of adding Avery Dennison to consortium.

Outcomes:

First proof of the relative importance of the four elements of weathering (Temperature, Humidity, Ultra-Violet radiation, Mechanical Load) by the Statistical Analysis Division.  A major step in developing the predictive model, due in FY13.

Development and deployment of the powered outdoor weathering sealant tester.  This will accumulate the critical outdoor weathering modulus data for validation against the predictive model.  

Impacts:

Consortium member continues to use the practice of simultaneous exposure to multiple weathering factors in new product development pioneered by this project.

Four companies have asked for a commercially available version of the SPHERE for sealant testing.  The commercial SPHERE was the only project selected for PHASE II funding of the SBIR.  

Standards and Codes:

PI is an active member of ASTM Committee C24 (Building seals and sealants).  Given the Award of Appreciation in 2010 by C24.

New ASTM standard to be issued, January 2011:  "New Test Method for Measuring the Time-Dependent Modulus of Sealant Using Stress Relaxation."  Balloted concurrent in sub and main committees and passed with no negative votes in June 2010 ballot. 

Initiated ASTM Round Robin testing (Inner Laboratory Standard (ILS # 0427) and completed precision and bias statement (ILS #0533) for new standard. 

New Test Method for exposure of a sealant to outdoor weathering with dynamic strain introduced to ASTM C24 as ASTM WK27077 for ballot in October 2010.