Mechanical Performance and Service Life of Structural Polymers: Pipes and Sealants

This project will develop and implement measurement science to characterize critical mechanical properties of polymeric materials used in pipes and sealants as a function of temperature, moisture, solar ultraviolet radiation, and mechanical stresses, and develop predictive models for long-term performance of polymeric materials used in pipes and sealants. These results will be built into new standards for these material applications.

Objective: By FY2015, to develop and implement measurement science that quantifies and predicts mechanical performance of structural polymeric materials, such as pipes and sealants, exposed to compound environmental stressors, including mechanical loading, temperature, moisture and ultraviolet radiation.  These results will be built into new performance test standards for these material applications.

What is the new technical idea?  Polymeric materials are utilized in critical construction and infrastructure applications ranging from high stiffness plastic pipes used in water infrastructure to high flexibility joint sealants used in commercial and residential buildings.  These materials must retain structural and mechanical performance characteristics over decades to prevent significant damage to, or failure of, the structures and systems in which they are contained.   When subjected to environmental stressors such as water, temperature, UV light, and imposed mechanical stresses, both pipes and sealants will experience both chemical and physical changes that affect mechanical performance.   These changes are often accelerated where dissimilar materials meet or similar materials are fused together.  Current test methods for measuring long-term performance of pipes and sealants typically utilize a single environmental stressor at elevated levels, but the in-service environment includes multiple environmental stressors applied simultaneously.   This project will develop measurement science to characterize the mechanical response of structural polymeric materials used in pipe and sealant applications that are subjected to multiple environmental stressors in a state-of-the-art environmental exposure facility. 

What is the research plan?  In order to develop and implement measurement science for characterizing mechanical properties as a function of compound environmental stressors, specific classes of polymeric materials, representing two extremes of performance, will be chosen to validate measurements and models.  The first is a high strength polymeric material, high density polyethylene (HDPE), which is used in natural gas and water pipe systems, barrier films, and geomembranes for landfills.  In particular, HDPE has been of great interest to the water, gas, and nuclear industries in recent years due to significantly (up to 60%) reduced installation and maintenance costs.   The second class of materials is soft rubbery elastomers, which are widely used as building sealants to prevent moisture intrusion and energy leakage.  National insurance studies show that water damage via moisture intrusion is 10 times more likely than fire, with 28 % of all property insurance claims related to water-related damage.  Additionally, energy efficient, air-tight buildings are not achievable without adequate long-term sealant performance.  Measurement science developed in this project will address the long term mechanical performance of these two classes of materials as a function of the four principal factors considered critical to the aging of construction and infrastructure materials: temperature, humidity, ultra-violet radiation, and imposed mechanical stresses.  The project is divided into three major tasks in order to develop and implement, via standards, measurement science for improved mechanical performance, as detailed below.

Task 1 - Characterize: Both classes of materials rely on specific underlying molecular structures to achieve the desired mechanical performance, and failure results from changes in molecular structure over time.  Task 1 will develop, prove, transfer and standardize methods to assess initial mechanical performance metrics (modulus, viscoelastic recovery, yield behavior and fracture properties) as a function of the preparation scheme. Examples of these methods include characterization of the viscoelastic nature of elastomers using stress-relaxation and control of slow crack growth resistance by local microstructure. 

Task 2 - Expose: Using methods developed in Task 1, material properties will be characterized during accelerated exposure to UV radiation, temperature, moisture, and static/cyclic mechanical loads.  These exposures, tailored to in-use environments, accelerate the degradation of mechanical performance.

Task 3 - Model:  Models will be developed and standardized as American Society for Testing and Materials (ASTM) and American Society of Mechanical Engineers (ASME) standards to predict the end-of-lifetime of infrastructure materials in regards to performance and safety.  For sealants and other soft materials, a statistically-based SPHERE data model will be used to develop predictions for outdoor exposure.   For crack-sensitive materials such as HDPE, fracture and contact mechanics-based models will be developed utilizing the essential work of fracture concept.  Failure times of pipe joints will be predicted utilizing a combination of analytical and probabilistic models.

Standards and Codes: In ASTM C24 Building Joints and Sealants, project results are being incorporated into ASTM work item WK20492,  Test Method for Measuring the Time-Dependent Modulus of Sealant Using Stress Relaxation, which has been balloted and will be issued in July 2011.   Another work item in the same committee is ASTM WK27077 which proposes to revise ASTM C1589-05 Standard Practice for Outdoor Weathering of Construction Seals and Sealants with the option of movement during weathering.  Project personnel have joined the ASME Boiler and Pressure Vessel (BPV) committee, and in conjunction with the NESCC effort, have started preliminary work to incorporate project results on plastic pipe into ASME BPV standards.

Recent Results:

• Output: Two industrial workshops on sealants were held that updated sealants manufacturers on technical accomplishments that need to be incorporated into industrial practice and active sealants testing standards.
• Output: Six peer reviewed papers over the last three years have been published that set the scientific foundation for, and are referenced in, new sealant standards developed in ASTM C24.
• Output: Renewed Phase III of the Sealants Service Life Prediction Consortium via continued Cooperative Research and Development Agreements (CRADA) with nine sealants companies, providing future support for measurement science focused on sealant service life prediction and standards 
• Output: Formation of Nuclear Energy Standards Coordination Collaborative (NESCC) task group on plastic pipe for nuclear applications, which will review standards gaps and develop a roadmap for key stakeholders and customers to identify measurement science gaps for plastic pipe in the nuclear industry.  Two presentations were given (November 2010 and March 2011) to NESCC members to report status on identified measurement science gaps to date.
• Output: Two presentations at American Society of Mechanical Engineers (ASME) Code Week (August 2010 and October 2010) on measurement science research on plastic pipe, which engaged ASME, a key stakeholder and standards development organization with substantial expertise and vested interest in the development of plastic pipe, in the NIST/NESCC effort.