Service Life Prediction of Filled Polymers Project

Objective: By FY2014, to develop and implement measurement science that will provide the technical foundation for  service life prediction of filled polymeric materials exposed to compound environmental stressors,  through quantitative tests and models applied to polymeric pipes, sealants, and electrical cable insulation.

What is the new technical idea? Prior efforts at NIST demonstrated the feasibility of quantitative service life prediction by linking the field and laboratory for an unfilled epoxy polymer in 2007. This methodology was based on characterizing the simultaneous effects of UV dose, temperature, and humidity in polymeric materials critical to coating and corrosion prevention applications. The next challenge is to develop similar methodologies for more complex and commercially-used semi-crystalline and elastomeric polymers containing fillers. Measurement science developed for these materials is also foundational for the enabling of service life prediction capability for structural polymer composites, another project in the Sustainable Engineered Materials Program. 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: (1) plastic pipe based on polyethylene blends (polyethylene, 800 million tons/yr, 40% of all plastic produced), (2) thermoplastic cable coatings for power transmission and systems monitoring, and (3) elastomeric sealants and adhesives for building environment enclosure ($50B/yr industry). Each of these industries lacks measurement science for quantitative prediction of long-term performance and each material is subjected to multiple environmental stressors over their lifetime.  Relevant environmental stressors include temperature, moisture, radiation (ionizing and non-ionizing), electrical fields, and mechanical loads. Standard test methods currently used for qualifying polymers in these applications may not be 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 to (1) 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) develop and apply models describing long-term material performance during environmental exposure. This research will provide the technical foundation needed for the development of test standards and models that will be ultimately be used to understand long-term reliability and determine the service lives of filled polymers used in these applications. 

What is the research plan?  Based on service life prediction problems in three prominent industries, the project team has selected specific classes of polymeric materials to support the development of measurement science and models for service life prediction. The first class of materials is semi-crystalline plastics, and more 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 cable coatings, whose function is to maintain power transmission integrity and safety of electrical circuits in power applications. The second class of materials is elastomers, which are widely used as building sealants to prevent moisture intrusion and thermal leakage. 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, ultimately leading to service life prediction capability that is applicable to other materials, both new and existing. 

Task 1 - Characterize: Both classes of materials rely on specific underlying molecular structures and active fillers (antioxidants, titanium dioxide and carbon black) to achieve the desired mechanical performance.  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 and yield behavior and fracture properties) and some select chemical and electrical properties.

Task 2 - Expose: Using methods developed in Task 1, changes in 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) as draft standards to help predict the end-of-lifetime for performance and safety of filled polymer materials. For crack-sensitive materials such as high-density polyethylene, fracture and contact mechanics-based models will be developed utilizing the cohesive zone model. 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). For sealants and other soft materials, a statistically-based model based on SPHERE exposure data will be used to develop predictions for outdoor exposure. 

In FY2014, research on plastic pipes will refine the measurement techniques developed for high-density polyethylene in FY2013 and focus on generating data to enable development of fracture models for pipe fusion joints. This data and models will be shared with project partners, specifically the Gas Technology Institute (GTI), Nuclear Regulatory Commission (NRC), and Dow Chemical, to verify results and validate improved measurements against previous methods.

In FY2014, the electrical cable research will focus on the implementation of improved test and exposure technology and performance characterization methods for accelerated laboratory and field exposures, including simultaneous exposure to temperature, moisture and radiation, approved by the NRC and the Electrical Power Research Institute (EPRI) in FY2013. In addition, new capability for electrical loading will be added to laboratory environmental exposure. Assessments of the results will be performed to determine if the simulated environmental conditions correlate to actual end-use conditions and if critical properties related to major cable failure modes are being properly measured.

In FY2014, the research plan for sealants will focus on: 1) predictive model development, 2) technology transfer and 3) standards development. In predictive model development, the primary thrusts include obtaining calibrated outdoor exposure data from several locations, continued acquisition of data from exposure on the NIST SPHERE, and refinement and validation of the statistically-based predictive model. The calibrated data from the SPHERE and outdoor exposures will be used to statistically quantify the uncertainty in the geographically specific predictions of the model. In technology transfer, a commercial version of the sealant SPHERE has been developed and introduced to a third-party industrial developer. The goal in FY2014 is to have members of the sealant consortium purchase working commercial SPHERES from that third party. In standards development, we will work with ASTM C24 to introduce new test methods, procedures, and practices, incorporate them into standards documents and submit them for ballot. The collaboration with Underwriter's Laboratories (UL) is planned to continue to grow, giving an increasingly larger scope for the application of this project's results.