Service Life Prediction of Filled Polymers Project

This project will develop measurement science and standards for service life prediction of filled polymeric materials used in the infrastructure and manufacturing.  Examples of  products and stakeholder industries that will significantly benefit from developments in this area include plastic pipes used for water and gas transport, sealants and adhesives used for building facades and environmental enclosure, and electrical cable insulation for cables used in power generation and transmission. Predictive models will be developed and validated using measurements of performance under interacting environmental stresses such as moisture, temperature, biaxial stress, electrical fields, and radiation.

Objective: By FY2014, to develop and implement measurement science that will give the technical foundation for  service life prediction of filled polymeric materials when exposed to compound environmental stressors through quantitative service life test standards for the end-use infrastructure and manufacturing applications in pipes, sealants, and electrical cable insulation.

What is the new technical idea? Prior efforts at NIST completed the first step in quantitative service life prediction by linking the field and laboratory for an unfilled thermoset polymer in 2007. This methodology was based on UV dose, temperature, and humidity in polymeric materials critical to coating and corrosion prevention applications. The next challenge is to develop service life prediction methods for more practical systems, such as filled semi-crystalline and elastomeric polymers. Measurement science developed for these materials is also needed to develop service life prediction capability for structural polymer composites. The complexity of the service life prediction problem significantly increases for these filled materials because their performance is derived not only from their molecular architecture, but also from the interactions between the polymeric matrix and the filler, which enhances the long-term properties of these types of (non-structural) composite materials. Therefore, the impact of microstructure on performance and the interaction with the environment and any cross-correlation between matrix and filler must be characterized and modeled during exposure.

Current service life prediction problems in three prominent industries will be leveraged to help solve quantitative service life prediction problems and develop measurement science for this class of polymeric materials. These industries are plastic pipe based on polyethylene blends (polyethylene, 800 million tons/yr, 40% of all plastic produced), thermoplastic cable coatings for power transmission and systems monitoring and elastomeric sealants and adhesives for building environment enclosure ($50B/yr industry). Each of these industries has a critical problem in quantitative service life prediction of performance and each material suffers multiple environmental stressors with minimal monitoring over this service life. There is continuous exposure to temperature, moisture, radiation (including ultra-violet and other electromagnetic frequencies), electrical fields, or ionizing radiation. Standard test methods currently used for qualifying polymers in these applications are typically not scientifically-based, and are generally useful only for detecting early failures, not for predicting service life or ensuring long-term reliability of products. Additionally, many qualification tests do not apply the relevant environmental stressors simultaneously, hence, knowledge of synergistic/antagonistic relationships between the environmental factors is lacking.

The new technical ideas in this project are (1) to develop and standardize protocols for characterizing filled polymeric materials as a function of exposure to relevant environmental stressors (individually and in combination), (2) establish the relationship between fundamental material properties and product performance during environmental exposure, and (3) based on data, to develop and apply models describing long-term material performance during environmental exposure. This research will provide the technical foundation needed for the development of industry performance and test standards that will be used to qualify the long-term reliability and determine the operating condition of filled polymers used in these applications.

What is the research plan? The project team has selected specific classes of polymeric materials, representing the two extremes of long-term performance, to support the development of measurement science and models for service life prediction. The first class of materials is semi-crystalline plastics specifically, bimodal high density polyethylene (HDPE). HDPE is used in natural gas and water pipe systems, barrier films, and geomembranes for landfills and has been of great interest to the water, gas, and nuclear industries in recent years due to significantly reduced installation and maintenance costs. Within this material class are also flexible wire coatings, whose function is to maintain power transmission integrity and safety of electrical circuits in power applications. The second class of materials is soft rubbery elastomers, which are widely used as building sealants to prevent moisture intrusion and energy leakage and as moisture seals in industrial modules. The project is divided into three major tasks in order to develop and implement measurement science for improved performance, which is embedded in standards and codes. This project will identify, measure, model and integrate scientific knowledge of degradation and failure into the development of standard characterization and accelerated testing for filled polymers, leading to service life prediction capability that is applicable to new and existing (e.g. remaining service life) materials.

Task 1 - Characterize: Both classes of materials rely on specific underlying molecular structures and active fillers (antioxidants, TiO2, carbon black) to achieve the desired mechanical performance, and failure results from changes in molecular structure or loss of filler activity over time.  Task 1 will develop, prove, standardize, and transfer methods to assess initial mechanical performance metrics (modulus, viscoelastic recovery, yield behavior and fracture properties) as a function of the preparation scheme. Other critical material properties in these applications include chemical and electrical properties.  Methodologies and metrologies for probing these properties will be developed for select materials, which will be chosen to represent the variety of materials encountered in real applications.

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. Both accelerated laboratory tests and field tests will be carried out. Standard protocols will be developed and implemented for exposure of select materials to relevant environmental stressors, including temperature, moisture, radiation, and electrical loads. The University of Maryland at College Park will contribute technical expertise and facilities for the ionizing radiation exposure of electrical cables.

Task 3 - Model: The results of Tasks 1 and 2 will support the development of physicochemical models connecting fundamental material properties, accelerated laboratory testing, and field performance results for material applications. Initially, statistical modeling and numerical calculations will be developed that will support the development of physics-based models. These models, after validation, will be introduced to the American Society for Testing and Materials (ASTM) and the American Society of Mechanical Engineers (ASME) as draft standards to help predict the end-of-lifetime to performance and safety for filled polymer materials. 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 high-density polyethylenehigh-density polyethylene, 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. For electrical cables, modeling will be utilized to validate acceptance criteria that are developed from condition monitoring tests and to predict long-term performance of new products. The developed cable models will be incorporated into test protocols currently under development in various standards committees, particularly in the Institute of Electrical and Electronics Engineers (IEEE).

In FY2013, the research plan for sealants will focus on refining the modeling of sealant exposure on the SPHERE, quantifying the agreement with newly-acquired outdoor exposure data.  Additionally, a new sealant, specifically designed for rapid degradation when exposed to the weather, will be used to greatly improve the prediction methods for in-service performance. The results from these tests will be shared with project partners, specifically sealant formulators, to verify results and validate improved measurements against previous methods. high-density polyethylene.

FY2013 will capitalize on the measurement techniques developed for high-density polyethylene in FY2012 to focus on service life prediction of pipe fusion joints. The results from these tests will be shared with project partners, specifically the Gas Technology Institute (GTI), Nuclear Regulatory Commission, DOW, to verify results and validate improved measurements against previous methods.

FY2012 marked the beginning of the electrical cables research component of this project, with selection of relevant cable materials that both experience aging challenges and are currently products in industry.  All associated standards were also reviewed. Preparation and development of measurement methods for examining critical initial and aged material properties (chemical, mechanical and electrical) was also completed. Work in FY2013 will develop and implement improved technology and performance characterization methods for accelerated laboratory and field exposures, including simultaneous exposure to temperature, moisture and radiation, and develop capability for electrical loading in various environmental devices. These testing parameters must be relevant to operational conditions and accepted by industry (e.g. approval by industrial partners). Analysis will be performed to determine if the simulated environmental conditions are correct and if critical properties related to major failure modes are being properly measured.

The results of this project will also support the structural polymer composite work, starting in FY2013, in the Sustainable Engineered Materials.

Recent Results:

Outputs:

• (Pipes): Nuclear Energy Standards Coordination Collaborative (NESCC) task group (Aaron Forster, chair) produced a report on plastic pipe for nuclear applications, which reviewed standards gaps and developed a roadmap for key stakeholders and customers to identify measurement science gaps for plastic pipe in the nuclear industry (submitted to WERB).
• (Cables): NESCC Task Group on Electric Cable Aging (Stephanie Watson, chair) produced draft report on gaps in standards, NRC regulatory documents and research for cable qualification, cable aging and cable condition monitoring metrologies. This output is a vehicle to assess gaps in cable condition monitoring and qualification standards in the nuclear industry and to gain support and research direction from end-users and potential partners for cable aging/condition monitoring measurement science research.
• (Cables): Proceeding manuscript and presentation for cable degradation research presented at 244th Fall American Chemical Society (ACS) Meeting in Philadelphia, PA.
• Fabrication, validation, and transfer of next generation active strain-based outdoor sealant testing devices to sealant industry, with incorporation into ASTM C24 test standard.

Outcomes:

• (Pipes): Incorporated NESCC standards roadmap for pipes into proposed ASME Roadmap “STP-NU-YYY: ASME Code Development Roadmap for high-density polyethylene Pipe in Nuclear Service” currently under revision at ASME.
• (Cables): Developed advanced measurement tools for polymeric insulation in electrical cables by combining spectroscopic, microscopic, and electrical measurement techniques for characterization of polymer degradation under exposure to relevant environmental stressors.
• (Cables): Collaboration with US Nuclear Regulatory Commission (NRC) was established ($600k/year for 5 years) with NIST for the development of condition monitoring methods for aging electrical cables in nuclear power plants.
• Impact:
• • C1735-11 Standard Test Method for Measuring the Time Dependent Modulus of Sealants Using Stress Relaxation was published.
  • 2012 revision of ASTM C1589-05 Standard Practice for Outdoor Weathering of Construction Seals and Sealants includes NIST generated data and peer-reviewed archival journal publications. It will be the first ASTM C24 standard to recognize and incorporate fatigue as a significant factor in the property changes over time of a filled polymer sealant material.

Standards and Codes:

(Cables) A task group in the Nuclear Energy Standards Coordination Collaborative (NESCC) is led by Stephanie Watson to assess gaps in and the need for new standards for electrical cable condition assessment in nuclear power plants. This task group will complete a final report in FY2013.  This standards activity also engages the Nuclear Regulatory Commission (NRC), the International Atomic Energy Agency (IAEA), the Institute of Electrical and Electronics Engineers (IEEE), and the Electric Power Research Institute (EPRI).

(Sealants) Chris White has been active in ASTM technical committee C24, which based on project results has issued ASTM E1735-11 and revised ASTM C1589-05 (details are above). The work of this project has resulted in mechanical movement being incorporated into all the relevant ASTM C24 standards.

(Pipes) Aaron Forster has been active in ASME Section XI activities on high-density polyethylene pipe code development in the ASME Boiler and Pressure Vessel Code meetings, and is participating in development of the ASME Roadmap for high-density polyethylene Codes and Standards.