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Accelerated Weathering and Service Life Prediction of Engineered Polymer Materials

Summary

Engineered polymeric materials perform critical functions in infrastructure systems, such as structural support (composites), corrosion protection of structural elements (coatings), weatherproofing (roofing, siding) or sealing the building envelope (sealants).  These materials experience changes in their properties when exposed to outdoor weathering, such as water, UV, temperature, and fatigue.  These property changes result in lower performance of the materials than assumed when the materials were selected and initially installed.  New metrology and models to predict service life performance of these materials are necessary.  Thus, tools for accelerated weathering data consisting of publicly-available laboratory and outdoor weathering data sets, measurements for laboratory exposure conditions, validated statistical models, and climatological-specific predictions based on the validated statistical models need to be developed.  To efficiently build the resource and validate the breadth of this approach, the project will focus on critical polymer chemistries for resilient infrastructure, construction, and transportation applications.

Description

Objective - Develop laboratory accelerated and outdoor weathering property-performance databases, traceable measurements, and validated statistical models for service life prediction of engineered polymer materials for resilient infrastructure.

What is the new technical idea?  

Resilience of infrastructure relies on all components performing as designed for the life of the structure.  For instance, a hospital was rendered inoperable after a wind event by the failure of the windows sealant used to isolate interior rooms from the outside weather. Polymeric materials are essential for transporting water, providing insulation for electrical cables, sealants for windows, etc.

As new polymeric materials and systems are developed by industry, reliability of historical performance records is not possible.  Thus, it is paramount to develop the ability to predict in-service performance of these products for facilitating the adoption of new materials and systems.

Performance under field conditions versus time curves must be known for each component.  To develop such curves, the field conditions need to be replicated in a laboratory.  Any test method or model used in a laboratory needs to be validated with outdoor exposure. Environmental factors for exposure that are taken into consideration are UV, humidity and temperature. Not all factors may be considered for simulation of field conditions, only those essential to structure application.  For example, pipes are usually buried and thus not subjected to UV. 

Outdoor data are then used to validate model predictions based on accelerated laboratory exposures.  Once a validated model has been developed, it can be used to predict a desired property change with exposure at other locations where weather data are available.
NIST has performance characterization measurements and corresponding exposure and weathering data, principally using the NIST Accelerated Weathering Laboratory, with well-characterized materials listed below: 

  • Epoxy amine (EA) was fully analyzed.
  • Polyethylene (PE) was partially completed.  
  • Polyethylene terephthalate (PET), and 
  • Polyurethane (PU) are in queue to be analyzed.  

EA and PU coatings are used to protect construction materials such as steel, from corrosion or deterioration. PE, the most widely produced polymer at 80 Million metric tons per year, is traditionally used for water/sewer pipes and window frames. PET is an important commercial polymer, with 34 Million metric tons annual production.  PET is also used extensively in making building envelopes impact resistant, thereby increasing the structure resiliency. For example, PET films are widely used to reinforce glass in the building envelope to counter threats to buildings and occupants, including natural disasters (e.g., hurricanes, tornadoes, severe wind storms, seismic activities), and manmade destruction (e.g., burglary, vandalism, terrorist bombings, industrial explosions).

For each material, data from both laboratory and outdoor exposures will be organized in a database and made publicly available.  The database would be used by industry as a reference when developing new materials or when developing and validating new prediction models. This project will establish a baseline performance for each polymer type (database) and will demonstrate the potential of model prediction for different commercially important classes of polymeric materials. 

One area of development required when using mechanical performance to monitor physical property changes is that of instrumental precision in measuring deformation. There is a wide range of plasticity in different materials.  Polyethylene will stretch 700% before failure, where PET or EA will stretch ~10%.  The current method of measuring plasticity of the sample is to measure the entire instrumental motion and assume that only the sample is deforming. For PE, this is a good assumption. For materials, such as PET and EA, this introduces a significant, instrumental-derived, experimental uncertainty. This project will explore a recently acquired non-contact strain measurement during tensile deformation using a digital image correlation technique to allow for direct measurement of sample displacement, thus, greatly reducing this instrumental uncertainty for low plasticity samples.  

Additionally, this project will establish characterization methodologies, statistical and physical models for predicting property changes, and validation procedures for those models. This project builds on state-of-the-art for demonstrating correlations between laboratory and outdoor exposure. The information, databases, predictive models will be actively transferred to industry.  NIST personnel will engage in outreach, communication, technology adoption and development of standards based on the outputs of this project.
 

What is the research plan?  This project builds on the success of the previous “Measurement Science Tools for Accelerated Weathering of Polymers” project, and will first complete the objective of providing a publicly-available database for weathering exposure conditions and material performance data for a thermoplastic polymer, polyethylene (PE). Ultimately, the project objective is to provide industry with a systematic methodology to develop service life prediction models from exposure data for a specific polymeric material using four steps:

  1. Design of experiment (DOE): identify the relative importance of each environmental factor contributing to degradation, such as UV, temperature or humidity;
  2. Exposure and characterization: conduct indoor and outdoor exposure experiments and characterize the degradation through measurements of performance, such as color change, tensile strength, and chemical changes;
  3. Database development: establish indoor and outdoor weathering/degradation data; record performance data (chemical, mechanical, optical) with associated meta data for a specific polymeric material.
  4. Model development & field data validation: develop a performance degradation model based on the indoor data. Then validate the model predictions against existing outdoor weathering and mechanical performance data

The following materials have been selected for NIST durability determination following the four steps above:

  • Epoxy amine (EA):. Create the initial database from the indoor and outdoor data that have already been collected, and validate the statistical model.
  • Polyethylene (PE): Complete the outdoor and the SPHERE exposure experiments, work with SED to produce a statistical material response model, and validate the model using outdoor exposure data from Florida.
  • Polyethylene terephthalate (PET): Previous exploratory work indicated that impact resistance imparted to the glass/film envelope system is strongly degraded by UV exposure.
  • Polyurethane (PU): This material will be studied in later years because it is a commonly used coating for corrosion resistance infrastructure and takes longer to degrade even under accelerated conditions.

 

Created December 1, 2017, Updated November 7, 2019